Lagging

[Alphaman]

Why do people in my area say its lagging down when it rains hard

[Grunty]

I don't know the origin, but this was written by a Captain Mercer at the time of Waterloo (1815)
"I had often longed to see Napoleon, that mighty man of war -- that astonishing genius who had filled the world with his renown. Now I saw him, and there was a degree of sublimity in the interview rarely equalled. The sky had become overcast since the morning, and at this moment presented a most extraordinary appearance. Large isolated masses of thundercloud, of the deepest, almost inky black, their lower edges hard and strongly defined, lagging down, as if momentarily about to burst, hung suspended over us, involving our position and everything on it in deep and gloomy obscurity; whilst the distant hill lately occupied by the French army still lay bathed in brilliant sunshine. "

[Quizmonster]

As long ago as the 13th century, one meaning of 'lag' was to wet or to bedraggle. I suspect the local usage you refer to is a hangover from that concept.
It seems to me that the use of 'lagging down' in Grunty's response above, just means the clouds were 'trailing down', just as 'lagging behind' means 'trailing behind'. The quote does go on to say 'as if momentarily about to burst'. That suggests to me that it has not yet even started to rain.
(Sorry, mate!)

[Alphaman]

Thank you Grunty and Quizmonster. Your answers help me no end.