Because you were overcompensating due to lack of proper acoustical treatments.
Bass Traps
I recently acquired some bass traps, four GIK Sound Blocks and two GIK 244 Panels with FlexRange. The Sound Blocks are on the front wall and the 244s are on the back wall. (There is not adequate room for the Sound Block on the back wall, hence their location.) Now I seem to have more bass than before. I had to reduce the level on my two Rel S/812s by one click and am contemplating another click. Anyone know why I have more bass with the traps than without?