Frank this may be a problem of the traces being underrated. If this is a demo or test circuit is the motor 12 v RATED or was itlast used in a 12 volt supplied circuit? If the later the motor windings probably saturate at about 2 to 3 volts thus The proper design of a driver in this situation is a constant current design. If you drive such a motor through a set of transistors the winding voltage exceeds the actual rated voltage and the current gets very high. The motor rotates and all seems well until you try to load it or run it at a low speed- then the motor overheats ant the smoke gets loose.

hope this helps Terry