We recently replaced a 12 lb entry level reactive resin ball that had a narrow (maybe 1/32") crack from a finger that ran about 3" heading away from grip center.  Our supplier replaced it based on pictures we sent and didn't request the ball be returned.  It was a conventional grip with drilled holes, no slug or finger grips.  All holes were well beveled and the pin was well away from the fingers (the crack did not go toward the pin).  I drilled the replacement ball from the original drill sheet.  The ball that cracked was drilled in November, 2014 and brought to the center with the crack in October.

Since I was not going to be at the bowling center when the customer picked the ball up, I used the original drill bit to check the fit of the thumb hole of the cracked ball.  To my surprise the bit went into the hole to about 1/2" below the coverstock and stopped.  I would have had to use a hammer to force it in.  My conclusion is that the core had swelled, putting enough pressure on the coverstock to cause the crack.   The original bits did fit in  the finger holes.

I talked to the manufacturer's Technical Director.  He confirmed that the different resins used in the coverstock, filler, and core could cure at different rates.  He said the coverstock had enough elasticity to absorb minor swelling.  He also said that they are constantly working on ways to eliminate cracking.  I'm going to return the ball to them for their analysis.

If you drill balls and have some cracked balls in the center, or when you get a cracked ball in, if possible check with the original bits to see if the core has swelled.  If you used a slug or sleeve you'll have to drill it out and can't do the check.  Please report any swelling back here.

I have deliberately not given the manufacturer's name because I doubt that, if this is a cause of cracking, it's isolated to one company.  I also told the TD I was going to post this here.  --  JohnP

They never stop curing is what has been posted before. So to what degree may vary depending on the process.

If production is such that items are always being moved out at a faster pace maybe less time is allowed to let the ball set which may lead to more bowling balls getting out and cracking that may not have happened a few years ago.

"From what I have seen of balls that crack, it's the coverstock that shrinks, not the core expanding. Why does the coverstock shrink? No idea."

If that had been the case with this ball the bit would have gone to the bottom of the hole.  --  JohnP

Not if the coverstock compressed the filler it wouldn't have. It would make the hole smaller.

From what I have seen of balls that crack, it's the coverstock that shrinks, not the core expanding. Why does the coverstock shrink? No idea.

same; but I would guess the cover shrinks because some formulations are designed to have air voids in them and over time the cover compresses to fill those voids.

same thing happens in certain mixtures of concrete and asphalts.

It sounds more like the drill bit walked as the hole was being drilled. This happens when the bit goes from the packing material to the core material and shifts off to the side slightly. The core is more dense and if the bit is dull, or the person drilling it is in a hurry, the bit can shift (walk) before it starts biting into the core material. I've had issues with several brands doing this.

If you can, run your finger or something into the hole and see if one side sticks out a little and the opposite side is depressed.

Balls can crack for several reasons, environmental (temperature extremes, high moisture in the area, rapid hot/cold cycles- near air vents) Ebonite's V2 was known for being easy to split; manufacturing (cover cures too quickly, cover is thin- Track Rule- had 3 of these crack as they were being drilled, or with lighter weight balls there is simply not enough support from the core/packing material to withstand repeated impacts with pins.

This is clearly my 2 cents. Lets look at whats going on. You take a round orb, drill 3 or mor holes in it at different angles and heave it at pins. The ball hits back plates, side plates, and whatever else is in the pit. I see some drop their stuff on the floor and the ball bounces like s basketball.
I have plenty of bowling balls over the years and only 1 cracked. Of course it was and still is my favorite ball, roto rsp. I have kept my stuff in my trunk year round, on floors and in bags. My buddy has had 4 split within a few years of each other. He keeps his on a rack in a controlled environment. He cleans them after every use. I rarely clean mine. I have minimal loss of reaction. When I do clean them, its a hot soap bath.
I personally think maybe those cleaners make the ball more susceptible to the constant abuse. Again, this is just what I personally have experienced and seen.
Also, just like everything else we buy, theres no money to be made in stuff that lasts. So why would companies fix it and lose out on sales?

So your hypothesis is that the switch grip caused the crack? My guess is the inner fit tight because the hole was drilled at a slight angle.

My friend had a dv8 misfit and one night he noticed a sound coming from inside the ball. All the holes looked normal, no cracks, the cover had no cracks, but when he shook it we could hear the sound. Seemed like something came loose and there's a void inside the ball.