First time poster here, longtime WS reader. There's something about this case that has really fascinated me, I'm of the opinion of LE that this was truly a stranger on stranger crime and that makes it all the more chilling.
I believe they intended to use some force to steal the truck but things escalated past the point of no return for poor TB. Maybe there was a physical fight over the keys, as some have said some of these newer vehicles aren't exactly easy to hotwire. I believe that despite DM not folding under questioning something like visible marks or bruises on his body made LE believe that he had indeed forcibly come into contact with TB. That is how they were able to charge him initially, along with SB and the "large man's" description.
Most of the stuff that makes people look for a deeper explanation (foolish use of the burner phone twice, parking at mom's house) can be attributed to colossal stupidity and freaking out under pressure or colossal arrogance. They certainly wouldn't be the first criminals to suffer from either condition.
Never underestimate some people's lack of "impulse control" or impatience, perhaps they were so peeved that they had chickened out with the large man that they took that extra risk. They just had to have that truck NOW.
Also don't underestimate the group dynamics of young males in such a situation, as a potential "Boss" figure DM would take the truck on the next test drive no matter what happened, afraid to lose face.
I'll be very surprised if LE doesn't find at substantial stolen vehicles, parts, or equipment on DM's properties. Like others have mentioned I believe that these thugs were emboldened by a series of other more successful crimes, and DM above others because of the apparent squeaky clean image he seemed to have amongst many who knew him (apparently).
The suggestion that they needed a stolen vehicle as a precursor to some other crime is also a compelling one. The picture of the excavator in the snow raises an eyebrow as well as yet unknown suspicions that had to have set the neighbours of the Ayr property on edge. I hope LE will go over that whole site with cadaver dogs and ground penetrating radar.
The fact the police have not released a sketch of the hooded suspect or the video of the vehicle makes me think they know well who that person is, as well as the person in the second car. LE must have gone through every call text and email of DM and that is how they probably got their leads. He obviously thought the use of a burner phone would insulate him from all that and I doubt he used that kind of caution with his "real" phone or computer.
I wouldn't be surprised if the hooded person and the driver of the other car have fled the country or has even joined TB in some shallow grave. Judging by how the whole carjacking seems to have escalated, with everything to lose, DM could have become desperate and tried to cut ties to everything. We now know for sure that there were (presumably legal) guns in the M household at one time.