Replying to Broken Drill Bits?

So me and some friends were building a rail this weekend. We used PVC pipes with dowels inside on a 2x4. We decided to drill some pilot holes through the whole works we wouldn't crack the pipe or split anything. We got like three holes drilled, but then we started breaking drill bits like crazy. We snapped like 3 or 4. We got the rail built, but I can't figure out what the heck was breaking those things? I've never ran into this before. Anyone else have this problem?