Well, rumour has it, that the reason for this is that drug gangs use fireworks to signal when they have renewed product to sell. Today, on this blazingly warm May day, two sets of fireworks have been set off. I can see most of the valley from way up on the hill where I live, but I couldn't work out where the explosions were coming from. Not from the local mosque, which often sets off fireworks when there's a wedding, but seemingly from the direction of the nearby estate.
I did a quick google on this subject, but was unable to find any mention of the supposed connection between drugs and fireworks, other than a couple of mentions in news stories, about drug factories being raided and fireworks being found. So perhaps it isn't true. I'd certainly like to see some evidence before just believing it.