If a big truck in front of your car begins to turn left, but its right turn signals are flashing, it is most likely that truck driver

It is most likely that the truck driver is preparing for a right turn.

A large panel truck driver making a wrong right hand turn can cause what is commonly known as a right hand accident. This would result into a crash from either the front or the side. It is worth noting that the bigger the vehicle, the bigger the area the truck needs to make a safe turn.
What this truck driver was trying to do is called the “jug handle” turn. His aim was to turn left first so he can get out around the corner with a wider clearance to avoid hitting anyone else on traffic. Whoever is behind would be able to stop, slow down and wait for him to negotiate the turn