Im not sure why, but.... ?

when i was walking my horse up to the arena, theres a tree near there. my horse spooked, and galloped straight to the tree. i stayed on, until my helmet hit a branch and i fell off. i landed on my hip, but i wasnt hurt. when we were practicing our canter, my friend wouldnt canter cause she thought she would fall. how do i get her to continue cantering? cuz her horse has a really nice canter, and is very gentle. hes an old warmblood. also, what could have spooked my horse? (shes a 23 year old thouroughbred, and doesnt spook much.)