To be fair, Sebastian does know what he's doing - he's painfully outclassed in terms of strength and speed, and he's quick to realise there's no way he could possibly pierce a monster's skin the way he usually would, so he focuses on sticking his knives in the vulnerable spots: the eyes, and at one risky moment when he has the chance, the roof of a monster's mouth.
It just so happens that that one risky moment coincides with Porthos coming to the so-called rescue. Sebastian's senses aren't sharp enough, now, to notice Porthos ahead of time, and so he whirls around to look his way at the sound of the snapping metal.
Stares, for one moment, at the broken gun-- not putting the pieces together immediately, but catching on, after a moment, that whatever that was meant to be, it was a failure on Porthos' part.]