Why is Time Machine using 180 GB of my 250 GB for backups?
Since OS X 10.7 (Lion), Time Machine (if enabled) keeps local snapshots on portable Macs. This gives you Time Machine-like access to recent backups, in case you need to restore a deleted or modified file (however you still need a proper external backup to protect against disk failure). This is what the Backups item is measuring.

This feature isn't enabled by default on desktop Macs (are you sure you have a Mac Pro, and not a MacBook Pro? One is a laptop, the other is a desktop), but can be enabled by running sudo tmutil enablelocal in Terminal, and disabled by running sudo tmutil disablelocal (you will be prompted for your password in both cases).

As noted in Apple's KB article on the subject, once free space on your disk falls below 20%, it will start trimming older local snapshots, and it will speed up this process if it falls below 10%. So if you do nothing, you should see your drive space return to approximately 50 GB free. You can continue to store new things on your hard drive without fear of running low on space, as Time Machine will just slowly trim away snapshots as you add new data.