It was only once I cued for trot that she started to spin out of control, and the events that led up to the bucking itself were very rapid-fire.
One rein stop. As soon as things start to escalate, pull her head to your knee, wait for her to stop, get off and move her butt, backwards, forwards, lunging, yielding, until she calms down and focuses on you. Then get back on, and carry on exactly where you left off.
Also, really pay attention to your self. Are you getting tense as soon as she acts up? She may just be feeding off you. To put it in perspective, I think of it as a fire. Young horses often "spark". They are testing their rider, or scared, or opinionated, so they try to pull something, not knowing what will happen. If you stay calm and/or react decisively in a way that stops the action, they cool off, and move on. If you tense up, get worried, afraid, panic or get angry, you effectively 'fan the flame', the longer this continues, the worse it gets, it often escalates quickly, and the worse it is, the harder it is to 'put out'. Pay very close attention to how you are reacting.