john deere 185 popped hydro fill cap and sprayed oil

Working my mower hard, went to go up a small hill, hydro lever was "shuddering". Wheels would start to turn then give up. under load the hydro pump oil filler popped and sprayed hot oil everywhere. What happened? thanks, geo

Last time that happened to me (new to me mower I did not check first) was water in the hydro fluid.