you need to recalibrate :-)
basically the hotend is getting steadily closer to the build plate.

Abs is a real basatrd to works with (unless you're sebastien :-) )
You have to get your calibration bang on and do it regularly.

Many many reasons to use pla instead. I calibrate approx every couple months or when I need to replace the tape. Good Pla doesn't warp, so you can get away with a lot of stuff you can't get away with for abs.
Performance wise Ive always found pla tougher, stronger and harder than abs. It probably depends what you are making, how you design it and what orientation you print it in.

But your current issue is calibration based :-)