Clindamycin - why do they say do not lay down for at least 10 min?

Because of the harm it can do to your esophagus if it isn't further into your digestive tract. Laying down allows a certain amount of reflux and that isn't the best thing to happen. Propping up in bed would help and drink a full glass of water after you take it.

The contents of your stomach may come up your esophagus a little bit and cause real discomfort like heartburn.