There are 2 plausible scenarios, one bad, one worse.
Scenario 1) There is major regulatory pressure that has come down and we are only seeing a bit of it from our vantage point. Dwolla seizure is just the tip. Many banks are refusing to accept wires from Mt.Gox account. I heard from OneFixit on IRC that his wire from Gox to Citibank never showed up, he called Citibank and they said they refused the wire and will refuse all future wires from Gox.
Gox is frantically trying to open accounts in other banks or in other names to disguise the origin of the transfers so it can resume sending wires out. How long before they get caught? This is the bad scenario.
Scenario 2) Gox does not properly segment customer funds, and without regulatory oversight the recent secular shift in market sentiment to more bearish tone has caused a major imbalance in wires in vs. wires out. They don't send wires out because they don't have a segregated pile of customer money. They spent all the money on hiring new people, or expanding, or speculating, who knows. All that matters is they don't wire out till people wire in. Gox is not a bank that has many loans out, where fractional reserve banking works, it is an exchange or brokerage where customer accounts should be segmented. Gox has been in seg fault for weeks and would be instantly shut down if there was any sort of regulatory oversight on their operations. This is the worse scenario.
Take your pick, they both suck. Or take plausible scenario 3, which is 1 + 2 simultaneously.
I suspected scenario 1 several weeks ago when the dwolla seizure happened. The halting of US withdrawals fits well into that scenario. It's possible they are scrambling to comply with KYC requirements, but this shouldn't be an issue for people who have already been validated.