Assuming this is the only path for hydraulic oil into the crankcase, the possibilities are the seal was installed backward, the seal lip was rolled or cut on installation, or maybe the bushing in the pump is worn to the point the seal can't hold it. The lip on the seal should be facing the pump, making it harder to install. If there are splines or a keyway on the shaft that could cut the lip, some black electricians tape wrapped on the shaft will protect the seal as it is slid onto the shaft. The lip of the seal and the surface it rides on needs to be lubed when installed. Was there a groove worn in the shaft where the seal was riding? If you go back into the pump to replace the seal again, take some pictures and post them.