Why do drivers quickly swerve back and forth when there's a caution?

I could go on about the answer all day. The reason is because they wanna get all the dirt of there tyres so they are clean for the restart, but more importantly its to keep temperature in them. Racing tyres preform best if they are over 90C. At this temp they can grip the track well with no under steer. If the tyres are cold on the other hand if there is a corner approaching almost straight after the restart and the tyres are not warm enough, the car could go into under steer and that's how some accidents happen because racing engineers are not letting their drivers know if there tyres are prepared for racing conditions. Hope this helped, and could still go on about it for a while haha