There is an issue that allows players to create greater rifts where the monsters only have very little life, regardless of the greater rift level. I've seen screenshots of a high level rift (60'ish iirc), rift boss only had 92,000,000 HP. So you can imagine that if ALL grifts (regardless of level) are as easy as a level 1 rift, things get out of hand pretty easily. The XP scaling seems quite absurd for lvl 100+ rifts, so that's how people are doing things.
The bug is related to the old "two-for-one/double rift" bug, but hopefully this will never enter a live patch!