<<Can anybody (or shorts) explain why this stock, after having fallen by 30%, is still loved by the shorts?>>

Quite frankly, the business environment sucks in the sector and will no doubt continue to be challenging for some time to come. USG is currently running 3-5x its lows from earlier in the year.

Put these two items together and you have quite an invitation to speculation. It's also possible there are other factors at work such as short positions as hedges (creditors and even perhaps so-called unrelated hedges, i.e. folks with what amounts to long exposure to other areas of the sector).

Whatever the mix, Quickmoves is hardly all alone in his pessimism.