What happened here?

That's what we call an oyster. Your character was running. You slid out of a sprint animation into the downward slope of that terrain. Then after sliding pressed jump with the momentum the slope gave your slide. That resulted in getting a further, faster jump. :)

I feel like you never pressed jump and instead the terrain just bumped you up out of the speed you had from the slide because the animation and height doesn't align with an oyster. But it definitely is something like that.