It's a bit of a mystery to me; first aid doesn't actually restore health, it just stops bleeding (it is only supposed to use one person on the squad to do it so if multiple healers were applying first aid that's a bit of a bug, but not a major one since the effects would be redundant). The only way to actually regain HP is done by resting over time at the hospital/clinic/safehouse, but in any of these cases there's a check that's supposed to ensure they can't heal over 100%.
My only theory is that there is some combination of factors that allows people to take negative damage from attacks, since there's no check against the cap when receiving damage (after all their HP is supposed to be going down, not up), but I'm not really sure how that would occur. Let me know if you see it happen again and maybe what you were doing just before it occurred. I do have a fix that should prevent it by just resetting anyone with too much blood back to the maximum at the end of each day but it's not a huge issue so I will probably just include it with other fixes the next time I do a build.