There are two plausible ways that such high losses could have been inflicted.
First, some of the defenders may have been struck down while out in the open. As described in that series, a party of British Legion dragoons was caught out in the open at the beginning of the battle. As also noted, some of the Loyalist militia were stationed in a redoubt, but were soon driven into the works. Here is another pension application that supports the latter claim:
William McGarity claimed, “we marched against Col[onel] Turnbull at Rocky Mount and drove them from their works into a house from whence we could not dislodged them” . None of the accounts I’ve read indicates that the Loyalists lost men while fleeing the redoubt, but it’s certainly plausible, especially if they were not near the gate, or if the abatis was difficult to cross.
Second, American Lieutenant-Colonel William Hill claimed that the British were in a frame house and that he expected that the Americans’ bullets would easily penetrate the wood, striking down the defenders inside. I pooh-poohed this description because Hill is not the most reliable source, and other sources agreed that the British were in log houses [see Note 4 in this post]. However, I recently came across a would-be pensioner that agreed with Hill. James McConnel claimed that, “the force of the British & Tories he does not know, these were in a large frame house stockaded round from which the Americans tried to drive them but without success as they were unprovided with cannon" .
So possibly one of the buildings that housed defenders was a frame house, and the defenders inside were quite vulnerable to the Americans’ gunfire. Hill claimed that while the frame house appeared to be vulnerable, the British "had placed small logs about a foot from the inside of the wall and rammed the cavity with clay" . But how could he know? The Americans didn’t have an opportunity to inspect the site until almost 2 weeks later (after the British abandoned the post), and this may reflect an improvement that was made after the battle. The gun shot holes from the battle would have remained in the exterior of the building, leading Hill to a false conclusion.
1. Letter from Charles Cornwallis to Henry Clinton, August 6, 1780.
2. Enclosure in a letter dated August 20, 1780 from Charles Cornwallis to George Germain. In K. G. Davies (Ed.), Documents of the American Revolution 1770-1783 (Colonial Office Series), Vol. XVI. Irish University Press.
3. Pension application of William McGarity, transcribed by Will Graves.
4. Pension application of James McConnel, transcribed by Will Graves.
5. William Hill's memoir, transcribed by Will Graves.