Question about defensive alignment

Sorry if this isn't post worthy or was maybe covered elsewhere. I have stayed off the board since the game and am not reading through all the bullshit to see if this has been discussed. Thought about putting it in the defense snowflakes thread but I imagine it'll get overlooked by this point. Anyways...

Against Army, the commentators consistently discussed a "bubble" along Michigan's defensive line and the QB would target that bubble every time with success. So against Wisconsin on their goal-line QB keeper, the announcers again showed the "bubble" and the QB dove through it with very little, if any, resistance from Michigan. 

So why the hell is that bubble there? Is it because we are playing three defensive linemen? Would the goal-line not be a good place to play MORE defensive linemen? Should a linebacker be looking to plug that gap and is missing the assignment?

I'm not a football savant but I have never seen this specific thing pointed out so regularly for any team before. Not trying to add anymore negativity or hot takes here, just looking for some football education on why this specific issue keeps coming up. Thanks.