Because his users has more funds in system than what's available funds in the wallet. He just gaveaway more funds to people than what he has.
and how can it happen? I mean the giveaways are like 100 satoshi or something right? Either somebody entered million or 10's of millions of captchas to free-withdraw his wallet out or there is a bug about giveaways in the script like another way around without captchas?