js you can also hit people repeatedly in the back, even if both players were on 1.8 or not. timing n stuff, and judging by Ampzi's skin, he probably knows how to do it right too (just aiming in the dark there).
things look sketchy with speed II and especially when you start running backwards, he can probably get the hit in the back easier because of that, and the fact someone also hit him in the back.
shrug dont run in F5 constantly next time, it impairs judgement, and makes it harder to to strafe. watch the video in 0.25x, looks less sketchier if you can focus more on his range and whats happening.