So I recently replaced the original bottom bracket (shimano bb7500) on my fixed gear bike. When I opened it up the non drive side bearing cage was completely disintegrated, that is all the teeth were gone and only a flat ring remained, almost looked like a washer. The drive side however remained intact. The bb shell was full of black crud - probably dirt mixed with packing grease. The bottom bracket itself is old but was in a good condition and the ball bearings and cages were brand new and of good quality. I installed it about 4 - 5 months ago. I ride daily and sometimes in wet conditions.
My question is what would cause that to happen?
Are there any preventative measure I can take, since the original bb now has some pitting on the spindle?