My response was mainly because the OP thought it was causing the problem and he had runoff from the roof. Normally, rain shouldn't be an issue but if you have runoff from a roof, that can work its way into places it shouldn't be. The Northstar is a pretty well sealed pump motor so I am actually surprised they had this problem. Both of my pumps are out in the open but our rain is usually mild.

The OPs problem could also be poorly manufactured seals. If those fail, then the water will get into the bearings and they too will shortly fail.