In one of the videos looking out towards the sea and to the right of the silos, you could see what appeared to be fireworks going off. Shortly (seconds later) the massive explosion happened.
https://youtu.be/CtuQEhTVxyw?t=3
Watch the video 15 seconds after where it starts for that view of the explosion. Look at the warehouse and what was happening over the roof of that warehouse.
It could be chemicals or canisters cooking off as well. It wouldn't be the first time that a fireworks facility or fireworks cooked off like that. Back in 2016 at the San Pablito Market fireworks explosion happened.
https://youtu.be/jaLS2aFDxZw In the video you can see the same tell tale signs even through the smoke as you can see in the Beirut Video footage.