I miswrote that... I meant, no way his car flips over that late at night and he walks away without a DUI unless he seemed absolutely clear. The fact that he wasn't charged makes the situation sounds 10 x better to me. Read above for my other posts talking about more plausible situations.
He likely fell asleep behind the wheel.
He's very fortunate because that kills a lot of drivers in the US.
What does "careless driving and failure to maintain his lane" mean? "Struck" another car sounds like maybe he was texting or doing something without paying attention. Checking his phone for something, etc.
In some states, the cops feel obligated to give someone a ticket if they crash. If your car flips over, you pretty much are guilty of careless driving and failing to maintain a lane.
As long as drugs/alcohol were not involved, it is the same kind of things that all of us.
Given what time the accident happened, I bet that he fell asleep at the wheel.