Could it be a male turtle trying to entice a female? They often swim in front of female, put front legs out and twitch them. If it is a male it is probably some variant of normal behaviour.

However, what little I know is mainly about tortoises so you might need an actual turtle person to help you. I was just interested because I am a neurologist and treat seizures. If you filmed it and it does look epileptic that would be very interesting in a nerdy neurological way.