Low spot on the trail. When it rains, mud pit.

The main path has a puddle there after rain. Source: British.

it looks like there could have been metal posts at these spots, maybe a fence to only let pedestrians through, and cyclists started going around

Could have been tree overgrowth that has since been pruned.

I do not think in the mud puddle theory. There is a moat right to the right where rain would drain. There is also no depression for holding any puddle.

I think there is a depression in the treeline and grass and also slightly on the road. I would say that there has been a broken/bend tree/bush laying in the way for over a year and was recently removed. There is no debre left, as the grass was also cut recently by a machine that also could have picked up any leaves.

The desire to overtake

...wait this isn't secretly a minelaying tips community is it?

Maybe shared pedestrian & bike trail? Bikes going around people

If this is in the US I would guess the city put a giant flashing "Share the Road" sign there for a while and everybody had to go around.

Looks like a bike track. Maybe that spot tends to be very muddy after rain, and therefore bikers drive around it?

It looks narrow down further so this would be a natural passing spot