Why do slow walkers swerve and drift?

[Sarcany]

M A V I C wrote:
But what I don't get is why they often swerve or drift when walking. For example, I come up on a slow walker on the left side of the sidewalk, and as I go to pass on the right they drift to the right, thus thwarting my attempt to pass. I've had it happen where when I then go to pass on the left, they move back to the left.

I dunno.

Why do Maryland drivers swerve rapidly and precipitously left before moving rapidly right and vice-versa, never signaling, always tailgating, and always traveling at unsafe speeds?

It's just one of those weird enigmas of the human-condition that you might see on any given day.