Once you start breaking spokes, there can be a zipper effect- because you rode some amount with a broken spoke, the spokes around it have assumed extra load and may have been overstressed. The next time you hammer up a hill or hit a bump the wrong way, one of those spokes pop, stressing the spokes around it. I had a wheel that started doing that and after the LBS fixed one or two they showed me how to fix the spokes and let me do them myself. I started marking the spokes that broke and, just as you observed, they were all in a row on the same part of the wheel. (This was on a 36-spoke wheel. I weigh 220 and was carrying maybe 30 lb. of commuting junk.)