The user reported them to hi-po not because they were 'hacking', but because they intentionally created a dangerous situation on a highway by stalling a car, which could have gotten multiple people killed.
It's similar to the guy that decided to connect to the turbofan with his laptop on a populated passenger airplane in flight.
There are so many interactions in a vehicle that the researchers have no way of knowing with absolute certainty what will happen whenever they twiddle a bit.
Yes, 99 times out of 100, things may go as expected. The 100th time one of the O2 sensors happens to be misbehaving and suddenly your car is accelerating unexpectedly because software.
(Warning: the above example is probably complete bullshit but listen to John Hughes talk about property testing the CAN bus and you'll get nervous about ever driving again.)
Not due to hacking, but a few weeks ago I had been involved in an accident because a Mercedes SUV randomly shut down in the middle of the Belt Parkway in Brooklyn, and I had to stop in a line of cars behind said SUV while the driver behind me didn't stop in time. Luckily no one was killed(one injury, not me, but my car got totaled) but had there been any involvement in their part on something like that, I would've lawyered up and sued Wired.
Link?
The only articles I could find were these:
https://news.ycombinator.com/item?id=5531679
https://news.ycombinator.com/item?id=8139905
https://news.ycombinator.com/item?id=9402336
But the first one makes some rather dubious claims and doesn't align with my memory of what I believe your OP is remembering. My Google-Fu isn't working out well for me tonight (in my defense, it's late, I'm tired, and I'm reading this for entertainment...).
Edit, also these:
http://www.cnn.com/2015/05/17/us/fbi-hacker-flight-computer-...
http://www.usatoday.com/story/tech/2015/05/16/chris-roberts-...
Among others. I couldn't find the HN discussion on this, though.
Edit edit: Found the HN links:
https://www.wired.com/2015/05/feds-say-banned-researcher-com...