Not running the engine for awhile could could cause it especially with todays fuel with ethanol.But there are also lots of reasons it could go bad.Could be one of many.The seals themselves could have been a cheaper brand ones etc etc I would replace them with oem seals only.They are fairly cheap.On the bright side the stator side is the easiest one to replace.Clean the seal area really good once you get the old one out.A very small amount of case bond on the outside of the seal before you install it is nice to do for insurance.Be sure to pack the seal with grease(reccomend pc grease) and to not set the new seal in to far.Setting them in to far will cause them to wear prematurely.And as said always confirm with a leakdown test.There is leakdown testers for sale on bf now for $32.Well worth the investment.Good luck