I had the same problem. Looking at the back of the switch told me the reason. The switch wasn't rated high enough for the current all those bulbs were pulling. The factory goofed by putting too many lamps on that circuit.
You could turn off a couple of the fixtures to reduce the current draw or do what I did and replace all the bulbs with cheap LED's from fleabay.
FWIW, figure the switch is rated between 10 and 15 amps and each bulb pulls about 1.4 to 2 amps.
The weak point is where you feel the heat.