The most significant events, in terms of US history, that I can think of that took place between 1770, when the black population in SC was 61.5%, 1780, when the black population in SC was 53.9%, and 1790, when the black population in SC was 43.%... was the
American Revolutionary War (1776-1783) and the adoption of the US Constitution (1789), which basically created "The United States of America" as a nation as it exists today.
If I were a betting man, I'd wager that a lot of black folks took advantage of those two situations to get out of South Carolina... which had a reputation as being one of the most brutal and oppressive of all the slave States... to get the hell out of Dodge while the getting was good... and before it was too late. Even more especially since one of the major sticking points of the formation of the US Constitution was the designation of some states as "slave states" and others as "free states". If there was ever a time to flee the US southern states... it was 1789, to get on the right side of the infamous "
Mason-Dixon line".
I'd also think that the seven years of chaos of the Revolutionary War presented other great opportunities to escape slavery. For one thing, the British were offering freedom to slaves who agreed to work for them, so that was as good a chance as any to escape. Another chance was if the owner of the plantation you were on was killed in battle and just never returned, or there weren't enough overseers left to keep you captive, because the able bodied men had gone off to fight, or if the slave themselves were offered into military service by the plantation owners themselves in their own stead, or if the Plantation was burned or occupied by the British. I'm sure the War presented a plethora of opportunities to escape South Carolina.