Has there ever been an uncommanded lowering of landing gear?
The answer to your question is, in fact, "yes" - but the circumstances involved demonstrate just how difficult it is to make an aircraft's landing gear deploy without being commanded to do so.

In 1985, CI006, a 747SP, experienced an uncommanded flameout of its #4 (right outboard) engine (something that particular engine was quite prone to doing) while cruising at FL410 en route from TPE to LAX.  An attempted relight failed, and a lack of corrective flight control inputs combined with a poorly-designed autopilot caused the airplane to decelerate and gradually roll into a steep bank despite the autopilot's attempts to counter the thrust asymmetry; the captain eventually noticed this and disconnected the autopilot, but made no flight control inputs of his own.

This, unsurprisingly, resulted in an immediate loss of control, and the aircraft rolled into a steep inverted dive, during which it exceeded mach 1 for an indeterminate period of time, plunging 30 kilofeet before the captain was able to orient himself when the aircraft broke out of the clouds at 11 kft and pull out of the dive, eventually levelling the aircraft at 9,600 feet.  During the dive and subsequent pullout, the aircraft was subjected to aerodynamic loads exceeding +5 Gs,1 which, in addition to causing severe structural damage to the aircraft (including tearing away large parts of the aircraft's horizontal tail), ripped off the body gear doors, broke the support brackets holding the body gear uplock hooks in place, and forced the aircraft's left and right body gear into the down-and-locked position.  (The added drag from the extended body gear [and from the aircraft's 29-kft maximum gear-down altitude] forced the flight to divert to SFO, having insufficient fuel to reach LAX with almost half of the aircraft's landing gear hanging out in the airstream.)

So, yes, it has happened... during an over-5-G pullout from a 30-kft supersonic dive in a 747.



1: The highest normal acceleration recorded on the aircraft's FDR was +5.1 Gs during the pullout from the dive; however, the absolute maximum G-forces encountered by the aircraft are unknown, as the extreme G-forces exerted on the aircraft, and the FDR, caused the latter to fail to record properly for large portions of the dive and pullout.