I have installed many of the regent along with other brand motion sensors and never seen one cause a fire however i have seen a couple overheat and darken case but never to point of combustion.
I would check possibilty of a power surge due to electronic nature of the motion componenet and also moke sure motion detector was not placed to close or fell into one of the flood light bulbs as I have seen this happen and almost catch a house on fire but home owner saw smoke coming from unit and shut it off in time.
Since many of these motion sensors are DIY projects i ner put it past a home onwer improperly mounting the unit and it cathcing the case on fire when it makes contact with a lightbulb.