Who is at fault for this car accident?

Car A is a Honda sedan, while Car B is a GMC truck. Car B drives recklessly, through tailgating, swerving in and out of lanes, and cutting off other drivers within feet of their car, including car A. Car A decides to pursue car B to get license plate info and to report the car to the police. Car A switches lanes to pass car B afterwards, where car B speeds up, merges, and slams on the brakes. Car A brakes but can't stop in time, and veers to the other lane to avoid car B, but without enough clearance, and part of the front of car A hits part of the rear bumper of car B. Is car A at fault for rear ending the car? Or is car B at fault for merging and then brake checking? This is from a legal standpoint, not based on who is morally right.

Your motives may be in question. There was no reason for "Car A", to try to pass,"Car B". The driver could have and should have dropped back. Its stupid to try to pass someone driving as described. I, and others would think something else was going on there. Not to mention the folly of pursuing a reckless driver. Sounds like a case of,"I'll show you.". A road duel.

In general, the car A would be in the wrong just because they were in the rear. Add to this the fact that they were pursuing the other vehicle, They may be able to call on witnesses that would support the story that truck B was driving dangerously, but they should be careful that witnesses might say that it looked like the 2 were racing, or that truck B appeared to be trying to avoid a road rage incident with car A.

This is a case where lawyers are needed

Doesn't matter what proceeded it. Lets take a look objectively - Car A is in the fast lane, going past car B. Car B moves over (without looking or looking and choosing to ignore that a car is coming up in a lane it is moving to).
car b is at fault if you have a witness.
if there's no witness and its he said she said, car B will always win.
just the way the world works.

From an insurance perspective, the insured is ALWAYS required to mitigate damages.
This means reduce the risk of incurring damages or stop damages from escalating.

Car A, knew Car B was driving recklessly…
Instead of Car A getting out of the way, they actually pursued the individual.
But not only that:. Car A has the information they "wanted" but even then didn't hold way back from car B

If you are Car A… It is very likely you voided your insurance coverage for this event.
Thus, you better hope that Car B coughed up there insurance and their insurance considers it their insured fault… Otherwise, you are going to be out the money for your car and the the other car.

Question: Who is at fault for this car accident?

Car B an idiot. Good drivers can see the idiots and avoid them. Good drivers don't need to "get back at" an idiot because we know that they WILL meet another idiot.

Car A is also an idiot. 99% of two-vehicle collisions is of two idiots meeting.

"… Car A decides to pursue car B… "
THIS is the cause of the accident. Car A is at fault by breaking many laws of the Vehicle Code, starting with road rage…

"Car A decides to pursue car B to get license plate info and to report the car to the police." And what were you going to do with that plate number? Unless the police witnesses the incident they can't do anything about it. What if you are trying to get back at the driver because you didn't like his bumper sticker or the color of his hair? It is not incumbent on you to enforce the traffic laws.

You should have just let the truck go and continue on its way. Instead, your anger got the best of you and you caused an accident. It doesn't matter if the pick up truck driver was driving recklessly. You escalated the incident and hit another vehicle in the back. Next time get a dash camera, this way you can prove that the other guy brake checked you.

TMI (too much information)
ONLY the involved insurance companies determine liability (fault). It does no good asking strangers who was liable.

Car A is in the wrong, you hit the truck in the *** end. Your at fault!

Legally, Car A would be at fault UNLESS he can prove that Car B unsafely changed lanes and cut Car A off, therefore causing the accident. But that would be pretty difficult.