I can claim no specialist meteorological knowledge, but I believe the
following to be undeniable:

1. When a thermal forms, there is to some extent at least an inflow of
air, to prevent a vacuum forming under the rising airmass.

2. It is extremely unlikely that the inflow from every direction is
equal and symmetrical. If Coriolis effects are likely to be swamped by
local initial conditions for the inflow, the resultant rotation may be
in either direction.

3. It follows that there will be an element of rotation imparted to the
resulting thermal bubble/plume/call-it-what-you-will.

4. In some cases at least, the resultant rotation may be enough to be
noticeable.

5. During the inflow phase, any resultant rotation will speed up, by
conservation of angular momentum, like a dancer speeding up a spin by
pulling in the arms.

6. This rotation is certainly visible in dust devils, tornados, and
other smaller local eddies. Even in the UK's usually mild thermals, one
occasionally sees bits of straw, grass cuttings, or dead leaves picked
up and whirling round. In the days of stubble fires (farmers buring off
fields, after harvest - now banned) bits of burning straw could be seen
in the rotating thermals.

[I have used "rotation" in the layman's sense that the air and its
contents are going round. There is a technical use of the word, if I
recall my fluid dynamics correctly, that the water in a whirlpool has
"zero rotation" which may also apply in thermals, for the same reason -
it is a mechanism where things can go round quite fast without a
significant input of angular kinetic energy. It is a feature of a
rotating fluid mass where the middle is going round faster than the
outside, which happens in whirlpools and when you pull the plug out of a
circular basin full of water. I propose to leave it there. ]

Chris N.