Carrie – I totally agree. Impossible to determine since we have not seen the horse, but both the things you mention are the likely culprits. I do not like to longe in a bit, because of the potential for harm, but I am paranoid. Prefer to use a cavesson, surcingle, crupper and (sometimes) side reins.
Or, the horse could be bored to tears.
It is never the horse's fault