I don't think there is enough information to come to a conclusion here, but I doubt your proposed failure mode.

Based on my experience with largish data in mongo, my guess would be that the database size was >> than ram and, due at least in part to mongo's design, when the master failed over the memory state of the new master didn't have the working set present in RAM. This lead to a huge inrush of disk IO resulting in all sorts of bad until the RAM state sorted itself out.