A bit of rust. How does it happen? Well, some metals oxidize and corrode over time. Then it falls apart. From the sharp angle of the break the bolt that secured it may have been too tight, causing a crack.
From the pics looks like it's been cracked quite awhile. I'd say just flimsy cheap metal and started cracking around bolt hole from the constant vibration and shaking as you go down the road. Most roads are awfully rough these days. Cheap thin metal been cracking for awhile finally had enough.
Rust and vibration and no post. You see up here in the Yankee states we have this thing called snow. It comes down this time of year. We also have this thing called salt trucks that throw salt on the road. Salt + metal = rust. Yes I'm being snarky, and for that I mostly apologize. But yes, just rusts out and since we do get that thaw / freeze cycle our roads are never in good shape, so the vibrations take a toll on the weak points.
Driver abuse! Lol. Looks like thin, cheap steel. Probably fatigued from repeated shaking during normal driving. 10 minute fix for the shop.
Well if they're motivated lol. Clean it up, run a couple weld beads and bolt it back up. Unless they've got a new bracket.
That's precisely why you should do a post trip, as well. That could have been fixed already, and you'd be rolling as soon as your 10 was up. Assuming it's a left hand turn out of the truck stop.