I walk a lot through grass and mud. Most of the times I am wearing water-repellent boots, but there have been times when I am wearing sneakers and I step into some wet mud or grass by habit and get them all wet. This person may be used to walking in grass/mud. Perhaps they work in the field, or enjoys hiking through wet grass and mud to find amphibians. So, that is one thing to consider: people experience different levels of discomfort when walking through mud.
You mention that he was wearing sneakers. Are you sure about that? Water-resistant boots make one extra comfortable about walking through wet terrain.
If the sidewalk was so broad that you could have two people side by side comfortably with extra space, then it is a bit weird but it could still be a habitual reaction to giving way to others.
Since I like walking on grass and I know that others might not, it is not uncommon for me to walk into grass to get out of the way. I also do this on the bike when the bike path is narrow because I am driving either a hybrid or an mountain e-bike and I know these handle the grass better than many other bikes.
Go team grass! 😁
Ah, well, if he was wearing sneakers then the boots theory does not apply. I am not sure. You have many possible answers already… The actual answer might end up being quite simple though. He may have realized the grass was wet a second too late, and by that point chose to commit as he was wet already.