To me it looks like they run out of fiat. Took out a good few USD over the last few months of 2013, no problem. A couple of days ago a substantially smaller withdrawal got stuck, due to insufficient KYC. Rubbish, I'd say. Some of the questions they asked (ie address of SEPA bank) are already answered by their db. Others make hardly any sense. Not unlike MtGox last summer. The delays got longer and longer, the excuses weaker and weaker. bitstamp came up with KYC, mtgox told people they could only transfer whatever tiny amount per day due to bank restrictions.
When the fiat came surging in, they were able to pay out, now, with people trying to cash out and presumably no fresh money coming in, they struggle... Only thing, they should have got the money at some stage.
For weeks I have been doing 5 digit USD withdrawals every single day at Bitstamp, no problems!!!
I did have to follow up on 2 extra KYC / AML questionnaires which took just 24 hours to be processed, other than that no problems.
Just follow up on KYC / AML questions and all is fine.