Inés asked me once what saturated ground does, and the honest answer is that it stops holding. Up at Toro Negro the rain gauge took about 31 inches in a few days; in Ponce the hillside under Mameyes let go before dawn, in 1985, with the houses still on it. At least 130 people died there, most of them asleep.
The digging at Mameyes was done mostly by hand, by neighbours and civil defence crews working in the rain because no machine could be got onto that slope safely. There's a memorial on the site now, and the ground above it was left unbuilt.
Kit, that's the mechanism exactly. Water filling the pore spaces between soil grains carries some of the load, so the grains press on each other less, and the friction that was holding the slope together drops. Days of rain, not hours, is usually what it takes — and in Ponce it came at the worst possible hour.