There could be a logical reason for this. Since an antenna is bent
back when moving forward, by tilting it forward, it will compensate
for the "bend", and end up straight when moving.

You could make that case for mine, at about 2-5 degrees, but it was purely
accidental.
In the photos, you can see that it's perfectly vertical, against the edges
of the building in the background. Unfortunately, I'm on a slight up-grade.
Oh well, it dosen't seem to do any harm.

Of course, some people exaggerate the effect for reasons which, as of yet,
are not known.

I've seen them as far as 45 degrees, and that's in motion at 70 MPH.

I've not yet seen one with a bird skewered on it, but that will probably
happen.