This can happen on pumps. It is just a nut on the end of a shaft after all. All nuts can work loose even if they are loctited in. No great job to pull the head off naw and again to check the tightness. They do get some hammer as do the other parts of truckmounts. Its worth putting half a day aside at least once a month and go over the whole machine tightening things up.
One point, if a joint is leaking, don't just tighten it as you are likely to make things worse. Loosen it then renew the thread tape, then re-tighten.