Don’t they have to mix it with some diesel or kerosene?
No.
Ammonium Nitrate (AN) has explosives properties all by itself. It is a very strong oxidizer so if you mix it with any kind of fuel it forms a binary explosive but if it is impacted by fire or sufficient heat it can mass detonate - see West Texas or any of the numerous stories of freighters carrying AN (fertilizer) that exploded with devastating results.
This incident allegedly involves an ammunition storage site so it is unlikely to involve AN. There is still a lot of old Soviet era munitions from the 60’s and 70’s floating around the Middle East in poor condition and I have read that Hamas and Hezbollah (with Iranian assistance) have started making a lot of their own munitions so it is either ordnance in poor condition or more likely some form of human error.
Could it be Mossad? Maybe, but regardless Israel will be blamed and many will believe it. They put them in refugee camps or other civilian centers for a reason and that is usually because Israel will not strike them (bad publicity).
Many years ago, when I was living in NM, they simulated a nuclear blast detonating a huge pile of fertilizer. There was an explosive charge that detonated it.
The blast that happened in Lebanon several years back, first caught fire and a vortex developed and spun the hot gases to point of a plasma. Then it suddenly went BOOOMMM!