Thermostat like they say. You got all the marks of it. The hoses pop because stat not opening is overheating engine to make more than normal pressure on them. No matter, if they pop quick then a sign they are old and need changing. At 100K they all should be new anyway or you're just looking at more overheats. You've already done it a time or two, engines will not take repeated overheats long without toasting their insides. Not blowing white smoke now but the very next time may do it.
The mechanic didn't catch it because it was working OK when he looked at it. Stats stick out of the blue when old but not necessarily every time. He should've taken the cue from your description and changed it though unless you tilted him toward not spending money until the problem definitely found. A mistake on stats, you change them BEFORE they go bad not after. Even one overheat under the right circumstances and the car could easily be bound for the scrapyard.
FYI, heater cores do not overheat and then stop, they stay doing it and most likely not that. The water pump can easily be checked for looseness in the shaft since right on front of motor. Why did mech not do that? I'm suspecting his skills more and more.......... ............
I change all belts and hoses, stat, and water pump around 75K to 85K, and again at 150 or so, I haven't had a single overheat in like 30+ years on 4 cars running pretty much 100% of the time. Or how you get cars to reach 200K+ with no issues at all. Driving them until they fail first is asking to walk home past a certain point......... .............. ........