I've never had it happen and I never totally thought about it, but I
believe I can explain why the aileron or flap would float up:

1) low pressure on the upper surface of the wing.

2) Mass-balancing on some controls at the leading edge (likely to be
AHEAD of the hinge/pivot point, right?).

3) If you start considering the control-surface as a free-floating
airfoil (on its own and not part of a wing), it will likely have a
coefficient of moment. And just like a winged airplane without a
downward force on the tail, that coefficient of moment will tend to
rotate the trailing edge of the "wing" (i.e. the control surface)
upwards and forwards.

Of course that's not to say any of this is applicable in all
situations, or that it will result in a predictable outcome... The
bottom line is that the best solution is prevention - do a CAC
(Critical Assembly Check) _and_ a PCC (Positive Control Check) before
the first flight, every day.

I tell you what scares me: We get in a rush on our launch-line, and
our students defer to the CFIGs for the takeoff checks when things get
hurried. We do have a preflight signoff for each glider (which
includes a PCC), that gets done every morning.... But I worry that
some of the students - especially ones that show up later in the day -
never get the "drill" down, and the launch-line rush is going to catch
up with us (i.e. THEM) someday...

--Noel