Distorted inlet airflow, or sometime reingestion or cross-ingestion is frequently the cause of a stall/surge. Crosswinds and/or r/t can produce the this
During this very transient event, the airflow through the burner is greatly reduced but the fuel flow is still about the same, so there's a temporary over-rich mixture that doesn't fully burn inside the intended space - thus the fireball. (There may be a series of them... organ-pipe effect you know)
If it doesn't last too long there may be no real damage, because the heat is so intermittent it doesn't raise the metal temperature much. But the mfr probably will require some kind of inspection to be sure.