We all have our little theories which will never be proven.
Mine goes something like this:
Diane is a habitual drinker/pot smoker. This is easy for her to do because her husband is at work in the evenings, so she has time to herself to imbibe after she gets her kids to bed. By the time Danny gets home, she is asleep and cannot be scrutinized. She has spent years grooming an image of the competent, intelligent, responsible individual. She gets up every day and goes about her business as primary breadwinner and super mom. Her husband is a lazy child who had not particularly wanted kids, but had agreed to the whole thing after Diane promised that she wouldn't let it change his life dramatically. In reality, this bargain becomes a very bitter pill for Diane to swallow and over time, creates a huge amount of stress (hence the drugs and alcohol) and resentment towards Daniel.
So much rides on Diane creating the image of the perfect employee, wife, and mother (probably a reaction to how her own mother failed her family), and she is brilliant at pulling this off. The admiration she receives for this usually compensates for the negatives. But for whatever reason, Danny has really pissed her off during the camping trip. Maybe she also caught him in a lie about when he actually arrived at the camp and where he was in the interim. Maybe she's just finally had it with having to do pretty much everything while he gets all the fun stuff. Who knows.
At some point during the drive home, she makes the decision to take the edge off her anger with some vodka and orange juice, which she purchased at McDonalds. Maybe this just doesn't sit well with her stomach so early in the day, so she smokes some pot in order to handle the nausea.
Eventually during this car trip, she realizes she has been busted by Emma, who has called her parents and told them something is wrong with her aunt. She can usually fool her own kids if need be, but Warren's kids are older, and this time at least one of them catches on to the fact that she is impaired in some way. She knows if she shows up at Warren's house in her current condition, her whole reputation/perfect act is blown to high hell.
So she drives and drives, hoping to sober up enough to be able to face her brother and SIL.
At some point she realizes she doesn't have enough time to do this. So she drinks a little more to get her courage up to commit vehicular suicide, with all the kids as unfortunate but necessary collateral damage. Or: perhaps she simply overestimates her ability to function under these circumstances and, in the midst of an alcoholic blackout, ultimately ends up mistaking the exit ramp for an entrance ramp, precipitating a head on collision.
The end.
I think that's just as plausible as any of the other theories I've heard.
