Wheel Bearing Caught Fire, Full of Oil. Why?

So, as I was accelerating up a steep 2 lane hill from a stop light, I had what I assumed was an enraged motorist in a minivan pull in front of me and draw me to a stop. While remaining in the vehicle as a safety precaution, a corpulent gentleman came huffing up and informed me that one of my passenger side trailer tires was on fire, only small flames now, but which he said was previously shooting large flames, being fueled by the remaing oil where the outer bearing had previously resided. I had just checked this hub closely that morning and glanced at it briefly 15 minutes prior. The axle nut was very rusty but there were no visible leaks. We had these tires off and torqued when replaced 1 week prior, so perhaps something came loose? The brakes were on the verge of bad, maybe they had been sticking? Literally 3 seconds before I was stopped I felt a sudden lugging so the bearing must have departed it's place of abode then, and I had been checking my mirrors often and hadn't seen any smoke. However, I am going on 40 days without a day off so maybe I just didn't notice. Luckily my hopper trailer didn't turn into a corn stove so I'm happy! Any ideas on why this happened?

 

I'm guessing it was brake related as I had just been braking to a stop on a downhill slope before I was stopped. I was told that if this happens again to this particular tractor and trailer to just walk away, so I'm guessing the lack of maintenance is correlated to this reasoning. I'm just glad the flaming hub and tires didn't go flying into the prairie wetlands I was next to. That would've been a sizeable fire. Would sticky brakes cause this much heat though?