I looked at the past weather function on the timeanddate website. Wind speed on Mont Ventoux today was less than for last year's race. So strong wind can be ruled out as the reason.
if there is a storm, and today is was likely, the wind can be strong up there...it is a place where weather condition can change heavily and become dangerous in about 5 minutes.
But you can predict that 10 minutes before and to stop the race if that is the case.. Today if wanst, so they could have go on with no problem....
You can even put riders in a car, and later start again starting with the same time you were losing....yes, it is half an hpour in a car, but you are not going to start totally rest then.
If today is the queen stage of le Tpur of course they would have done full stage.