J. P. Gilliver wrote:
From time to time, my desktop icons rearrange themselves, into
columns on the left. It isn't a big problem, as I have EZdesk, but I
am curious about two points:

1, obviously - what causes it?
2: they aren't continuous columns. My normal arangement is around the
edge of the screen, more or less. If you do an auto arrange, it puts
them in a solid column down the left side, then a solid row in the
second column, and so on. This self-rearrangement does mostly that,
but there are gaps. (No overlapping/hidden ones, I think.)

I've seen it do it, very occasionally, and you'd expect it to start
top left, and go down in stripes (rows) until it's placed them all.
But no, it starts at where it would finish - i. e. part way down an
incomplete row some way across the screen, and goes UP and then left,
i. e. (a) completely the reverse order (b) it knows where it's going
to finish/start!

Very odd.

Sometimes you can just delete the shelliconcache and reboot. The
shelliconcache will rebuild itself automatically.
Many times this solves many icon problems. Simple, safe, easy and free.
Worth a try while waiting for other replies.