I can attest to what Mike says re dirty dryers.One morning  friends dropped in 
for coffee .The dryer was going with a large load,when it caught 
fire.Fortunately we were home or there could have been a great loss.
After putting the fire out,I dis-assembled it,[ it was approx. 3 yrs old] and 
found  a lot of lint;and this was an upscale dryer.