Lots of comments here focusing solely on the beer pong aspect of frat parties. You guys aren't wrong: there's nothing sexist about beer pong.

The use of the word 'frat' really is the difference here. Beer and beer pong are the obvious connotations with frat parties. Scantily clad women, roofies, and rape, are some others, even if those associations are unfair. Why in the world would you associate your company with something like that?

The party itself isn't some egregious, unforgivable mistake, but the fact that while under investigation for gender discrimination, it does something like this, illustrates to me that Twitter is pretty oblivious to the problem.