Question
Got more info for you:
1) The system has an electronic timer control system.  There is NO pressure switch/tank.  Plumber proposed adding that to prevent future problems.  He also proposed extending galvanized about 5 feet from pump (where pipe goes up & out of house) to prevent heat build-up from melting PVC in case we get zone valve not opening again.
2) The joint was Threaded PVC into galvanized (threaded) elbow.
3) The plumbing co. owner inspected the pipe and showed me a normal PVC threaded connection to show that mine had "melted"... the pipe & threads in the connection that 'popped' was thinner/smaller than the normal one.
4) The sprinkler system was running on the 7th & final zone when it 'popped' (6 previous zones=2 hours of watering time).  I had gone outside that morning and saw zone 7 running, and looked at the timer system to identify how long each zone runs, etc.  Zone 7 runs 20-25 minutes; not sure how long it had been running when I saw it.
5) The plumber says the damage could have been done during previous zones not coming on and causing the pump to heat up due to running but not having any sprinklers open; i.e. no water flow.  He thinks it then broke loose during zone 7 (which was running normally) due to compromised PVC.  Is it possible the connection became so weakened it just broke loose?  Is there any way I can determine if the plumber was at fault for putting the PVC/metal connection together wrong?
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Followup To

Question -
I just moved into a house with an irrigation system using a basement well as water source.  The plumber I had in the house for other work noticed a leak coming from the PVC pipe leaving the well pump (the system had just been turned on that day, so the leak was still a trickle).  He replaced the PVC coming out of the pump with metal pipe and metal elbow (about 6 inches total), and reattached the vertical PVC pipe that takes the water out of the house to the irrigation system.  One week later the connection (metal to PVC pipe) came completely apart and our basement flooded.  The plumber blames a sticking zone valve, saying this would cause the pump to stay running constant and overheat, causing the connection (only about 6" from the pump) to overheat and 'melt' the PVC connection (whatever adhesive was used?).  IS THIS A REASONABLE EXPLANATION?  Should they be responsible for the failed connection since just "fixed" it 1 week ago?

Answer -
Mathew,
 I'm sorry but the stuck zone would not have anything at to do with the connection coming apart. I find it hard to believe that the pump could overheat from a stuck zone. The pump has a themal breaker in it that trips before the pump overheats to keep from damaging the pump.

Perhaps theopposite of a stuck zone would be more likely to cause the pipe to blow. If you had a zone that did not come on and the pump kicked on if it is wired at the clock and not on a pressure switch/tank. What kind of connection did he use to get from pvc to meat pipe. A rubber boot or threaded conection. Let me know, Jim

Answer
Matthew,
If there was a zone that did not come on, then the pump was probably pumping and the water had no where to go and the pvc pipe will melt sometimes before the thermal breaker in the pump trips. I've seen where the pvc will swell up like a balloon from the heat. The plumber was absolutely correct in trying to get you to use a metal pipe out of the pump, although copper might be better. So I would think that the plumber made a good connection, but when the pvc melts any coonection is likely to fail. You may also want to check the program on the clock to make sure that all unused zone have zero time on them. The pipe melted for some reason and it was either a zone that did not come on or an unused zone (like number 8?) that had run time on it but does not exist. As for the pressure tank etc.., it is a good idea, but will be expensive. Jim