As evidenced by the lip and garter spring, the crankcase oil seals are intended to keep oil IN the crankcase, not keep water OUT.
Keeping water out of the crankcase is the responsibility of the low pressure seals, the V-packing, and the plunger O-rings and back-up rings.
Cracked plungers or improperly installed/damaged plunger o-rings allow high pressure water to be forced down the inside of the plunger. This hp water will easily lift the oil seal’s lip from the crosshead.
Because your issue occurred immediately after the service, I would suggest reexamining the plungers, their o-rings/backup rings and the proper orientation of the water seals.