You can trademark anything that isn't already trademarked, but you have to defend the trademark if you want to enforce the trademark.

Trademarks are a lot like patents, the due diligence when filing is on the filer for the most part. The agency only checks against the filings. So you can get a trademark for Hackathon, but if you tried to enforce it you would lose if there are prior uses by others.

You could get a Trademark in an alternate context. Like if you decided to combine a marathon with a machete and have a run through the jungle. That could be a new use for Hackathon that would be unlikely to cause confusion in the market.

That said I'm sure they will send some C&D's and scare some people in to not using the term.

The "Hon" trademark for a Baltimore restaurant springs to mind. Where the owner tried to extract money from the city of Baltimore, but she ended up dropping the trademark after community backlash.