My best guesses:
#1. They have a daily limit on the account, so they can't pay more than €xxx a day to withdrawal requesters
#2. Their € account is empty and they need to wait till people fund their MtGox account with €'s so they can pay the withdrawal requesters.

To explain 2 a little better: Suppose you start an exchange and 10 people fund their account with in total €1000. They buy 10 bitcoins for €100 each. Next day the bitcoin price explodes and are worth €500 each. If 5 people want to cash their win, it would require €2500 to make this happen, but ... there is only €1000 available. Two withdrawals will take place, three will have to wait...

MtGox claims it's #1 that is causing the delays. You might have another idea.