I successfully wound my first 0.05mm wire guitar pickup rewind, but one
curiosity- anyone know the reason?

This sort of rewind needs winding on a demountable former and transfering,
as a hank of wire, into a trough. On demounting, there is a distinct banana
shaped bowing to the hank. All I can think , there must be a twist in the
wire on the spool or some bias on left hand traverse versus RH traverse.
Despite pulling the wire off the supply spool tangentially rather than
axially.

I just thought of another possibility. For this sort of very fine wire,
instead of a final delivery pulley I use a tiny PTFE lump on a small bar. A
hole in the PTFE squashed to form a sub-mm slot. Then a 15 foot run back to
the spool (and a light felt slip clutch) to allow for any snatching. Because
of room arrangement , to get a 15 foot run , the winder and so PTFE is set
at an angle of 25 or 30 degrees rather than straight for-aft. Perhaps that
puts a set on the wire.