Actually, my curiosity was stronger tonight x)
I activated the blur in my parameters, and shot a replay to get a look.
Here is the fact :
The motion blur is created by the bloc "
Depth of field" !
Settins (french names, I can't be sure for a translation) :
Distance focus : 10
Taille de la lentille : 0.05
I added this bloc to my MT and it looks great
Hope this will help in the future