According to this article the authorities think that a blown tire caused a broken axle:
KMOV) — Traffic on eastbound I-70 in Foristell, Missouri is backed-up after an accident involving a tractor-trailer that caught fire.
Authorities said a car hauler headed eastbound on I-70 blew a tire that caused an axle to break and another tire to blow. As a result a fire broke out on the hauler that destroyed four band new Ford Fusion cars. Eastbound 70 was shut down for approximately an hour and a half.