apologies if this is obvious but if the tyre blows off the rim (for any reason) the forces are immense, and after a few times this can turn a tyre that was about the correct size when you started to one that is a really baggy fit on the rim.
If/how quickly this happens depends on how well made the tyre is; in the case of a M+ there is a coil of steel wire that is inside the tyre bead, normally about three turns. The ends of this are (amazingly) only secured by virtue of the fact that during the tyre moulding process, rubber infuses the tyre bead and this bonds the turns together so that the coil doesn't just unwind. However
a) if the rubber hasn't infused fully and/or
b) if the tyre flexes in the bead area repeatedly (which can happen if the tyre is run too soft for example, or refitted repeatedly) and/or
c) if the bead sees an abnormally high tension force (which happens if the tyre blows off the rim)
then the tyre bead wire can start to unwind and render the tyre a slacker fit on any rim.
Bottom line; the tyre can start to blow off the rim for any reason but once it has blown off the rim a few times it may just carry on doing so.
So it is possible that the tyre was a bad fit from the start, and that they tyre wouldn't have worked on any rim. From your description of it, it certainly wouldn't work now. However in view of the fact that I have seen this exact happenstance before, I think it is quite likely that part or all of the problem is caused by the rim, in that tubeless rims like the Mavic one (which is UST compatible) have little or no lip on and this seems to cause problems when fitting M+ tyres especially.
cheers
