Why am i disqualified ?

Hello,
Sometimes, while i doing a great race (and I'm leading) i have got the bad surprise to see that i finally finished last with an minute and a half late.It looks like a penalty but I've got no message during the race and when it's the case i 'm doing the drive trough.
Have you got the same issue ?
What am i doing wrong ?

Thanks in advance

 

If there was a mandatory Pit Stop in this Race and you didn´t do that, you get a Time Penalty at the end of the race.

 

Every time I m doing my mandatory pit stop, that's why I don't understand.

 

Maybe you don't do it properly. As far as I remember you need to change at least two tires and do it during pit window.

 

Changing Tires is necessary if you have to do a mandatory pit stop.

 

I remind people of this during the race start often / still a third doesn’t pit at all - another third just grabs a liter of gas, and then literally 2-4 people end up in the top spots with 2/3’s of the field flashing 1:30+ next to their times.

What’s really too bad is - if you don’t pit at all - the game needs to yank you from the field and disqualify you. I end up battling much slower cars who never pitted, basically every race with a mandatory pit.

 

That's it ! I'm disqualified when I 'm only refuelling.
Thanks all for your answer .