Running

Just that by the look of it, judging by other spikes.

why the spike for the sub 3 zone? Other than it being a popular objective

It'll be entirely down to it being popular.
My only standalone marathon was 3:15:29. If I felt I was near to sub 3 then I'd have gone for it. But I don't think I'd have had enough knowledge to know if actually a 2:56 or 2:58 was doable etc. so I'd go for the nice big target.

Same for the big 3.30 and 4.00 spikes. And the drop off at 3.01, 3.31, 4.01 etc

Having thought about it, if my only standalone marathon was 3:00:29 instead of 3:15:29 then I reckon I'd have found that 30 seconds to go sub 3 that I didn't bother to look for to go sub 3:15

Take into account people training, the Strava population,the runs that get published etc. That selection looks like it's skewing what is almost a normal distribution.