Spa pump keeps blowing apart

[blockhart329]

My sister has a spa and the housing on the jet pump blew apart. The threaded inserts just pulled out. I drilled through and installed nuts on the back side. When I brought it back online, the sliding gate valve after it blew apart. I replaced the valve and turned it back on and everything worked just fine. I turned it off to fix a tiny leak in the plumbing. When I turned it back on, the pump housing completely blew apart. What could cause this problem? It is a reiforced plastic housing on the pump.