maybe it was sent to the port recently to get shipped, or maybe recently docked there and stored. Maybe an undercover sabotaged/set it off, maybe timed bomb, no one knows..
Maybe you should stop inventing "facts"?
Look, that stuff was confiscated 7 years ago and put into a storage place large enough to hold it (which happens to be within the harbour area, no surprise there). How much of it has remained in storage is up to anyone's guess (it makes for great bombs and there are quite a few groups that do that as a habit in that area of the world). It doesn't help that the whole Lebanon has become a place of warmongering, and in everyone's mind it's Allah's will anyway if that stuff blows up or not one day, so noone cared.
Eveybody has known for years that it's going to blow up eventually, piecewise or in one blow. The big rubble pile still standing you see in pictures are/were the countrie's whole grain reserves, so expect a massive famine in a few weeks.