I vote for your connectors being worn or damaged in some way, what Apexspeed says is right. You are getting localised heating at the connector hot enough to melt solder, if the whole motor got that hot it would be scrap!
A bad connection and the associated current high power brushless motors use will warm up the connector enough for it to unsolder. I guess the 10.5 was ok as it is lower power, and/or the male part of the connector on that motor is in better condition.
G