For me, it was because my mountain bike gear was being washed on cold as heat screwed up the Eva foam in the knee pads and body armor. The jersey also got washed cold, so bacteria never got killed off. When it got hot, the heat would activate the smell, but it's hard to tell when you are going fast down a trail.
Once I started washing everything possible on hot, it stopped a lot of the smell.
I assume people are doing that with non-mtb clothing and getting similar BO results. So it's quite possible it's also not necessarily them, but their clothes.