Tri-Attack has an equal chance of inflicting either paralysis, freeze, or burn. I think what happened here is, when the system rolled to decide if an ailment would be inflicted by Tri-Attack, the roll came up good for Burn, but rather than stop there the system decided to keep rolling for the other statuses. The next roll came up good for Freeze, so it inflicted this as well, overwriting the Burn it already gave. This probably indicates that the logic for Tri-Attack will need to be looked into to improve how the status is determined so only one is applied.