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.
Think a little more about it. You have a trading platform. Someone gives you 1000 Euro, anotherone 10 BTC. No trade yet.
Now the exchange rate is rising from 100 Euro/BTC to 500 Euro/BTC. Now a trade happens, 2 BTC for 500 Euro each.
There is also 1000 Euro and 10 BTC on your trading platform, as before, nothing really changed for the trading platform (but for the fee).
Offers are possible only for 1000 Euro. Your failure is the 2500 Euro, which does not exist for changing.