Crankshaft pulley unbolted on its own?

[Pyramid]

Okay so I made a recent post about the 240sx I bought and how it needed all those gaskets.. but, the valve cover was dry.. After looking at it I spotted that a belt was slanted weird so I followed it and noticed the Crankshaft Pulley was slanted AF! I also saw the bolt sitting in the pulley.. So my question is, wtf could cause it to become loose an come undone? She said a month ago is when she got quoted on the gaskets (Front engine cover, Oil pan gasket, valve cover and a front crankshaft seal) and then last week she was getting ready for work, started up the 240 and went inside then came out and saw oil everywhere n turned it off.. Never started it after. So what could have caused this to happen? A bad gasket caused separation?

Pic:

[SoundEfx]

Was any work done to it before?
Asked because it seems as if it wasn't torqued down correctly or something.