Don't know the SH gear but as several have mentioned, the over center feature seems common to all. In the LS series, the pivot bolts in the gear can get a little stiff with age, grit, and lack of lubrication. The damper strut can get stiff when it's cold, which is designed, I believe, to prevent the gear from vibrating out of the over center position but can also inhibit it snapping into the locked position. And the gas spring that is the actuator itself can lose up to 50% or more of its rated force, which means the gear isn't being pushed into and held in the over center position as forcefully. When you touch down with a little bump...auto-retract occurs!

For LS (1, 4 and 3 that I know of), the combination of weak gas spring and stiff damper strut (both exacerbated by cold) are bad news. Stiff linkages just make it worse. Extend the gear and then look at it from below. If you can push the struts into the locked-down, over-center position that last little bit by hand, there's a problem. Fix that first. Some of the same may be true for other makes. My ASW gear easily goes over center and has a positive lock at the lever. But when down on the ridge, I have to wedge something into the gap by the lever to prevent a big bump from knocking the lever out of the detent and allowing the wheel to drop with an alarming noise.

Chip Bearden