Did a search on this but could not find much, apologies however if this has indeed been raised.
Quite often I get booted to spectate for being 'idle on track'. Try to get back in and perhaps a few corners later same again. It has also happened after several laps racing, though mostly it happens shortly after leaving the pits.
I don't have a great connection, with a fairly high ping (500+). Could this be the cause?

Only happens on server 1 (regardless of circuit). Does not happen on BnJ.

If you have a poor connection and the server isn't seeing you move (i.e. you are moving, but the server doesn't get the information that you are) then I suppose this might happen. I'm not sure what can be done other than you finding a better connection?

This same problem happened to me and I had a near perfect connection, there was no lagg at all.

It happened to me too. and it was on the last lap when I was going in the 4º place somebody told me it could be a connection problem but I didn’t have any lag on me...
anyway it's weird. here i let the replay so you can see it.

relpay_idle_ctra_race1

The server replay would be more revealing, then if you had any lag on the outbound socket it would show up, whereas a local replay cant be 100% certain not to contain any lag you may have had.

It is most likely to be lag that causes this. On race & single seater servers it detects movement of your car along the path node of the circuit, it needs to see movement in every 20 second window of opportunity. On B&J it simply times each sector as 90 seconds.

It is plausable that when you initially [re]join a race I might have a bug by not resetting the timer, if this is so it would be fixed on single/race as soon as the lights go green +/- 5 seconds and on B&J by the first sector.