There's a million and one conspiracy theories on this forum.
How about this one...

1. Gox really did have a bug in their bitcoin software.
2. They need to fix it like they said they do - and need to stop withdrawals to fix it.
3. 99% of bitcoin investors are either paranoid freaks or paranoid fat cats and so they all freaked and decided to sell loads of their coins - because they all read way too much into the hype and hysteria.
4. The price on Gox therefore tanked.
5. They will fix the bug and go right back to where they were before this happened.

Hmmm... almost tooooo simple?