While the “fertilizer” angle is obviously a cover story, it could well also be the case that was an IED facility - after all that is the center of their warfare laboratories. It needn’t be either/or, it could be “and”.
Netanyahu complained before the U.N about two years ago, about the three sites in the Port of Beirut that were Iranian/Hezbullah missile storage places. The site labeled #1 in his complaint, is exactly where the explosions took place.
The solid rocket fuel in the missiles is what caused the second and third explosions. The third explosion was similar to a Directed Energy Weapon, in that the explosive power was directed upward, probably to avoid as many civilian casualties, as possible.