How old is the house? It may have 60 degree C (centigrade) rated wiring. I'll bet those (newer) fixtures require at least 75 degree C rated wires in the installation instructions. If this is the case, it seems the wires simply got too hot for the insulation. It is possible to splice newer (90 degree C rated) wire in the system, but that would most likely be a job for an electrician.
If the wiring is new already, the fixture is getting too hot. Is the fixture installed per the manufacturer's instructions? There is something wrong causing too much heat. Have the light(s) cut off unexpectedly for no apparent reason? If not, is also possible the temperature limit switches are defective and the fixture is overheating without tripping them.
Either way, I would not use the fixtures in their present configuration for obvious reasons.