Assuming that the saw is not ancient, the most common cause of crank seal failure is freezing the bar and then gassing the saw to try to get the chain moving again. It only takes a few seconds of the clutch slipping at full throttle to heat the crank and melt the clutch side seal.
This is why some people say to never cut ice with a chainsaw. They may not even realize what happened, but you can melt a crank seal in 5-10 seconds trying to free an iced up bar and chain. I learned this one the hard way.