I have had balls get a crack completely around them while they were on the floor in my house. What do you think causes this?

They crack because the coverstock is placed under tension as it cures and shrinks slightly around a noncompressable core.

Here's a small list of thing's that start cracks:
(The tension in the cover material is what causes it to propagate around the ball.)

Temperature Changes
Gravity
chemicals in carpets
chemicals in upholstery
the glue used to put the inserts in
the glue was placed on the cover stock not the filler
too much glue was used
the holes not beveled
bad plug job
the bit they drilled the holes with was dull
They Drilled the holes too fast
you let the ball sit too long without rotating it.
the bridge was too narrow
the holes are too close to the pin

Thin covers may be a factor also

A newer thought going around is that balls are shipped out before they are fully cured and that may be a factor also.

Undrilled balls just sitting on the shelf in proshops have been know to crack.

Eons ago, when ancient man first played the game of bowling, like the Greeks, the Romans and the Egyptians, there were gods for everything including one for the new game of bowling.  While the game of bowling survived the centuries, the name of the ancient god (or goddess) of bowling was forgotten and that ancient one displays his/her displeasure with that insult by causing random bowling balls to crack.

This is the best explanation I have ever heard on why bowling balls crack.  Kudos Gary!!!