“Something had to trigger it”
Wikipedia indicates the tons of ammonium nitrate must have been a mixture which was intentionally created to make it an explosive.
So yes, I think the explosive mixture needed a detonator to make them explode, so there was a trigger other than a simple flame.
The ammonium nitrate mixtures are good commercial explosives partly because they are safe unless used with a detonator.
AN-—Diesel oil-— stick of dynamite.