I am to believe that any unit under fire on a mine will trigger the mine if it hasn't been triggered by the type of unit itself.
I don't think so. Many mines get finished under fire, as we could also see in the last tournament, and I can't remember that they would blow up regularly. Actually I can't remember them blowing up due to this at all.
It's either a weird bug (it's CoH2 after all) or a very very unfortunate chain of RNG for Latch.