According to dpb :
Chris Lewis wrote:

I don't think it necessarily flew straight at all. It merely
needed to be oriented the right way at impact. Besides, given how
much the trim probably weighed, it'd fly relatively straight at least
for a while.

I don't think that a likely scenario at all -- not impossible, but
unlikely. Being light, if it got crossways to direction of travel, it
would flutter and slow down extremely quickly imo.

MDF trim is quite heavy. Thrown lengthwise with no initial
sideways component and minimal flex, it'll go quite a distance
before destablizing. But destablizing is inevitable (if it doesn't
hit the dirt first) and will probably be quite abrupt. Destablizing
will occur earlier for wider trim (width versus length ratios
count!). That didn't look particularly wide.

Try throwing, say, a 8' piece of 1x10 lengthwise. It's possible to
keep it flying straight for a moderate distance without too much
difficulty. Try the same with a 8' 2x4 - it'll go farther before
destablizing. That piece of trim would fly a lot better than a 1x10,
and worse than a 2x4.

Directional stability can sometimes be rather complicated.

For example, I have a small/fattish (low and slow) rocket that
behaves rather wierdly. By the rules, it should be too close
to unstable and potentially dance around the area at launch. CP
is too far forward relative to CG.

[It was a very early rocket, I didn't know any better at the
time I built it.]

Under power or initial coast it flys dead straight - super reliable. So
reliable, in fact, that I use it for rocket demos at elementary schools.
Low and slow enough it can be fired with the largest Estes motor
available (maximizing the ooh and ah factor) and can be easily seen by
even the kindergardeners, and yet it's so light it can't hurt anyone and
will reliably land within even a small school field.

If it fails to deploy the recovery (chute), it falls flat
(broadside) after peak altitude. (which is a plus for safety)

Rockets generally can't do both. Fellow rocketers have difficulty
believing it when they see it, and yell at me for flying an unstable
rocket. But there's never been a hint of instability under
boost or coast.

[Estes made a rocket which was essentially an inflated
mylar "sausage" balloon. It does fly straight (sorta ;-),
and just floats down when the boost quits.]

As long as there's no cross push on the stick of trim, it can
be modelled as a short object with a lot of mass and a very
small cross-section. Once it gets some sidepush, it'll start
to go off course, and the effect will rapidly increase because
of the high side area.

It also occurred to me that with a kickback, there might
be a significant rotation along the long axis - spin stability.
It'll rapidly decrease given the surface area, but initially
it'd help it go straight.

Spinstability would be very difficult hand-thrown.

What I'm now
suspecting is more likely is that it actually fell mostly given the
description of the upper story being added...

Makes both hand-thrown or saw kickback more plausible at that distance.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.