My '05 did the exact same thing on the first pull!
We have two grandsons living back with us now, so I thought I'd go out and give it
a try, you know, just in case...
Pulled it hard, didn't want to move at first, and "pop-snap"!
It opened, but wouldn't return, and the front latches wouldn't relatch.
Took it back to Ford, and they said it was a 'Poorly Engineered Design".
Replaced the assembly, including cables under warranty.
They said the sharp bend, or angle of one of the cables had something to do with it.
The cable gets kinked in that sharp bend and requires excessive force to pull.
Thus, breaking the mechanism inside the lock cylinder.
When I got her back, I got out there one day with some of that white lithium spray
lube and lubed the HE(( out of every moving part on that tonneau. Latches, key cylinders,
center hinge point, and everything else I could see that moved.
Now it works as smooth as glass! Another example of the manufacturer going too
cheap on the grease (or lube)!