Hard to say because of all the different variables.

The biggest issue normally is coming in behind the head pin. Since you think that isn't the case, the other thing I see is incorrect ball roll that has a lot of deflection

Pocket 7-10 has been around since the mid to late 70's.  I once left 4 in one game in 1980 in Portland.  However I agree there has been a resurgence in recent years, with even high rev power players leaving them.  Basic cause is the high entry angle on a lighter pocket hit that normally would not leave a 10, but would be subject to the 5 pin either taking out the 7 or coming up just short. 

We first saw this when oil was being shortened because of the major carrydown problems on urethane lane finish.  Even the polyester balls would make a sharp move off the end of the oil.  As soon as the carry down pushed the break point a foot or 2 longer, 7-10 would start to pop up for us strokers on light hits.  When the oil patterns got longer with the advent of resin balls, the leave became a bit less common.  Not sure what has caused it to become so common again.  I dont see it very often on really long, or really short patterns, but around 38-42 is where they pop up for me.  On house shots I see it as soon as the heads force me in a bit and I still have to go around the puddle in the middle.  Once the track totally breaks down and I can move even deeper they start to go away.

 I see it as somewhat a product of the modern era.

 You see Norm Duke do it every so often. His ball hits the breakpoint, goes into the roll, but not at a steep enough angle. It still looks decent to the eye, but it enters the pocket incorrectly, deflects more than usual, and leaves a wrap ten while also sliding the five in front of the seven.

 Roll it out too steep, get no deflection, and leave a solid nine. Roll it out a tad weakly at the wrong angle, get too much deflection, and leave a seven-ten.

If it was just me I would have to think it was something I was doing but it seems that it happens a lot to all style of bowlers. Is it the more aggressive balls ? It just seemed that in the distant past you got a 7-10 when you hit the head pin flush. Maybe lack of maintenance on the pin spotters ? It happens in both houses I bowl in. 

 I think it is just a byproduct of the modern era and the way the shot is played.

 Everything is going fine, then the fronts get just a little burn, the ball reads just a tiny bit early, and rolls at the wrong time and at the wrong angle.

7 10s are pretty rare when using my Hy-Road with my style (another of many reasons its my go to ball).  I really have to nail the head pin just right and even then much more likely to get a nasty non 7 10 split instead (Hy-Road cuts down on those as well).  Now my red razor is a split machine.  Do get more 7 10s with that ball.

(edit: with my high axis tilt too much entry angle is seldom an issue and I rarely get washouts as well even on bad misses but I am in the minority of bowlers with modern equipment).

I leave them from time to time but more often will leave 7pins (right handed) usually because of entry angle and the amount of friction downlane.  Too many times I see the headpin hit the side wall and cross over in front of the 7pin and fly across the lane…… sometimes to thankfully scout the 10. Other times the 10 is gone and the scout just sweeps the lane.  Usually happens on the very very fresh backends or much later in the set when the lane is really starting to hook.

Back when I was learning the game there were considered to be 5 types of pocket hit strikes.  The light shaker type of strike where the head pin went to the wall in front of the 7.  The so called blow out 5 pin where the ball finishes strong enough to drive the the 5 out.  The half pocket strike where the  6 takes out the 10 off the wall.  the flush 10 in the pit strike, and the 4 pin trip.  Without going into the geometry of the posssible leaves on each of these hits, the likely leaves on each type of hit changed over the years as shells got stronger and back ends got drier.  We use to see 5-7, 4-5-7, and 8-10 fairly commonly especially on slick fresh conditions.  As entry angles got stronger, the deflection changed dramatically on the blow out 5 hit and the weak 10 hit.  That is when the blow out 7-10 started to appear.  5-7, and 4-5-7 became much less common as did the 8-10.  In addition the 4-9 and solid 9 started to appear more and more on the trip 4 pin hit.   

Blow out 7-10 has been around for a long time, but I agree it seems to have made a bigger resurgence in recent years.   I think one explanation may be the trend to longer patterns, but still very strong back ends.   I see more power players leaving them than was the case a few years ago.