Sherco kick starts break now and then especially the 290's and as noted when cold. I have done one on an '07 290.
There are different ways to kick them. Some stand up on the pegs and others bring the whole body weight down standing on the ground, not just extending the leg.
It is best to closely control the kick and concentrate to avoid the kick back that I think causes the breakage.
Look at how other riders start them, allowing for weight age skill etc.