The seeds of these plants were present in the soil all along and were released from their dormancy by the heat of the fire.
The fruits of these plants were transported into this area from outlying, unburned regions.
The seeds of these plants were near the surface of the soil at the time of the fire and were protected by their seed coat and dormancy. Their dormancy was broken by their new exposure to the light when the covering plants were burned away.
All three explanations probably contributed to the presence of these plants after the fire.