Gunter Herrmann wrote:

It can also be assumed that the seats did not become loose at any
point - they were loose (not properly fastened to the floor) to
begin with.

No, you can install a screw incorrectly by applying more torque
to it. This one may break.
Then the other ones might be overloaded.

These seats are not held down by screws.

The problem has been blamed on accumulation of soft-drink residue in the
"seat-lock plunger" mechanism.

http://blog.apex.aero/cms/wp-content...at-plunger.jpg

See this:

http://www.cnn.com/2012/10/05/travel...lem/index.html

=============
Something called the seat lock plunger mechanism can "get gunked up over
time with people spilling sodas, popcorn, coffee or whatever and that
can affect that locking mechanism on the ground that locks the seat to
the floor," airline spokeswoman Mary Frances Fagan said Friday.

11 witty and sage airline 'gunk' retorts

Apparently, worn locking pins can get stuck when food and beverages
spill onto them, allowing seat rows to come unhinged, she said.

American earlier said saddle-shaped clamps installed to hold the seats
down were put in backwards.

While the clamps may have played a role, the soda and snack gunk now
seems to be among the culprits, Fagan said.

And while that explains what happened to the three American flights that
experienced loose seats, it doesn't explain why the problem has affected
only American flights in recent days, or why it's happened so many times
in such a short period.

Besides presumably cleaning out the gunk, Fagan says American mechanics
are "taking extra steps to ensure that the seats do not dislodge from
the track."

That includes installing industrial-strength metal ties as a backup,
airline spokeswoman Courtney Wallace said.
==============

I would say that it doesn't seem possible that this seat-lock mechanism
can be partially engaged. Seems that it has only 2 states - locked and
unlocked.

And if locked, it doesn't seem possibly to unlock without a special
tool.