It's a good topic that hasn't been fully explored. Here's a fairly recent FG article on the relatively high IFFB rates in the minors compared to MLB. Might give you some more food for thought.

I've thought that hitters who pop up have a hard time laying off high fastballs and an uppercut swing, but that's just conjecture on my part. I hope you keep looking at the issue.

By the way, Joe Mauer is the king of not hitting IFFB - 2.2% of FB for his entire career!

For most people, popping up means that you were late on the swing by just a whisker. Most batters use an uppercut swing,

To hit a ball squarely, you are really aiming at that middle inch, shown by the narrowest of seams on the baseball.  Hit the bottom of that inch and it is a (hopefully) long fly. Hit it on top of the inch and it is a screaming grounder. In between... dead solid perfect... .

So in a normal swing trajectory, if yo are under the ball, you are late.  Makes sense to me that minor leaguers would have a higher late swing rate. They are younger and still developing, and I think the hesitation of youth .... but is there any way to measure the speed of a swing? 

For most people, popping up means that you were late on the swing by just a whisker. Most batters use an uppercut swing,

 

To hit a ball squarely, you are really aiming at that middle inch, shown by the narrowest of seams on the baseball.  Hit the bottom of that inch and it is a (hopefully) long fly. Hit it on top of the inch and it is a screaming grounder. In between... dead solid perfect... .

 

So in a normal swing trajectory, if yo are under the ball, you are late.  Makes sense to me that minor leaguers would have a higher late swing rate. They are younger and still developing, and I think the hesitation of youth .... but is there any way to measure the speed of a swing?

There must also be some selection bias in there. Minor leaguers with very high rates are probably not spending a lot of time in the majors and get taken out of the group. Is Gordon’s low rate a reason to be encouraged?