The Bungalow

So, on every pair of PJs I've bought for Martouf recently, there has been a warning tag saying something along the lines of "Clothes should be tight fitting. Loose garments are more likely to catch fire." I don't understand what that means. Why is it only on PJs? What are they catching fire on? Or do children's clothes have a tendency to spontaneously combust? Seriously. What is going on with that warning?