She is simply doing it because she can. She has found out that you cannot hold her at the canter and she bolts off into the wild blue yonder. It will get worse and she will start bolting as soon as you start her on the longe. I have seen them go from this to bolting when they are just on a lead.
I like to always start a horse to the right. This is so they do not just start running past me and start themselves. If you are not physically very strong, you may need to put a chain over her nose or under her chin for a while.
I start with a short 12 - 14 foot rope. [I hate flat longe lines.] I use only the trot for several sessions if a horse has already learned to bolt. I teach a horse to stop when I pull it hard around to face me. Then, I change directions and pull it around to stop every 2 or 3 circles at the trot or any time the horse pulls even a tiny bit on the longe line. I want slack in the line. I do not want it to touch the ground but I do not want it tight either. Every time the horse tightens the longe rope, I pull it around and stop it and either start it over or reverse it and start it over.
If I start to pull a horse around to stop it and it does not stop, I jerk the rope hard several times and pull it around hard again. I get them so I can barely pull on the rope and they stop. I DO NOT use the word "Whoa!". I say nothing. I want the horse to watch and respect the slightest tug on the line. I want the horse to have absolute respect for me and that rope and never, ever tighten it.
I have had horses that I could longe in hand over complete jump courses and do numerous trail obstacles on the end of a 25 foot rope. I move them up faster with a smooch and teach them to drop down a gait with an "Easy".
Until they are this obedient at a trot, I do not canter them. When I start cantering them, it is only for 1 lap and I drop them down to a trot or pull them around and stop them and change directions.
In a few minutes, I can have almost any horse paying attention and working on a loose line at the trot. A couple of sessions more and they are working like that at all 3 gaits. If you are very demanding and very exact in what you accept, you can have any horse working like this on a longe line.