This is the infamous "Ultimate Noob Boss" made by Iceguy. It is now back with a new name that implies it can be used for more than boss fights. It has quite a few attacks and is very customizable. Read the comments in the .asm file for details.
Yeah, it was strange physics bug with how gravational acceleration and terminal velocity works, when the sprite jumps, it goes up as normal, but when it starts falling, it accumulates its downward speed quickly as if it was forced into the ground by telekinesis.