It is mostly a matter of "feel". Basically, I am really good at threshold breaking in a car. I took several driving courses when I was in law enforcement, some in the military and practiced on my own quite a bit. So, when I learned to ride a bike my initial assumption was that it would just translate directly over. I discovered when I crested a hill and there was a truck stopped just over the crest that my breaking skills were not as good as I assumed, and that I needed practice. (No harm done. I locked up the rear wheel, let it go and scared myself with a wobble.) I suppose part of the difference may be that you are manipulating two break controls instead of one. It seems like the bike shifts weight to the front if not differently than a car at least more perceptibly. In a car, you don't have to and can't change the level of breaking on the front and back wheels separately as the car's weight moves forward. When you first hit the break on a bike the back tire has a lot more traction than it will in a fraction of a second. So it is easy to lock up. Also, when you threshold break in a car and a wheel locks up it is easy, even automatic to back off and let the wheel start turning again. If you do this on a bike when the rear wheel locks up, like you said, you risk a high side. One good thing is that with the weight moving to the front tire, unless you are on a loose surface you would almost have to try to lock up the front wheel. I guess, I really can't explain it other than to say, my experience transitioning over to a bike was that my breaking skills in a car did not come with me. While I understood threshold breaking in theory, I had to start all over teaching my hand and foot to actually do it.