Cloud is just young and needs more training.
It's normal for horses to get excited when the hrose in front of them runs, so cloud , being young , got especially excited. He could also have some issues witht the saddle, which when he canters, become more than he can tolerat. or it's jsut the movement of the saddle, or it could be you got nervous and started gripping hard onto his sides.
there's a number of things it could be. but the bottom line is cloud is basically a baby, who needs more training before being used to canter in a group, or ridden by children or inexperienced riders.
Don't blame him or think there are three seperate issues here. they are really one and the same; more time training with a clear leader.
Sounds kind of general, but it's true. Just don't look at it like I need to fix these little problems , 1,2,3. What you need to do is get him in a round pen, work him without a saddle so that when you say "forward", he goes.
Work on leading him correctly, and for that you can take some training from videos such as Julie Goodnight, or clinton anderson, or ? many others out there.
and you need to round pen him with the saddle on, so he gets more experience cantering with one on. and be very sure the saddle fits correctly, so that it is not pinching or otherwise causing pain, 'cause that'll for sure make a youngster buck, and that's a bad habit to allow to get started.