There are a couple mods that do that when they get tired of the chat's overwhelming faggotry/repetitive stupidity. It's usually a higher level chat mod. such as Spriker, Mew, Badgerz, Gary. Sometimes I do it too. Means to stfu. Though, it is rare.
The chat runs on Flash, so make sure your Flash player is up to date.
Those might be some of the reasons. I have deleted some of Dark's and Lorenzo's chat posts, but not recently.