When riding over a bumpy road surface, the clip that is supposed to hold the 705 in place can bounce enough to release the 705.
I think you are doing something wrong. I swap my 705 between my road and MTB. It has taken some fairly rugged descents on the mtb, plus a few occasions
when I've smacked bits of my body against it (ouch) while failing to make it up climbs. Never has
that clip released. In fact it takes a reasonably solid push to release it and given the mass of the lever, the chance of that happening through inertia seems
like zero to me, unless you ride off the top of a building.
Mine requires almost zero force to move the lever to release the 705, I would estimate just a few grams of force, the lever bends extremely easily. I wonder if there are different versions of the mount around. Mine is a one piece design that came in the box with the device. I'm not sure what exactly I could be doing wrong with it, I mean it just slides on and the retaining mechanism clips into place. I honestly don't know how exactly it has come off, just that twice it has come off when I've pushed the lap button at the top of a hill climb. My guess is that it had already come loose at that point, and pushing the lap button just gave it the final push to come off the mount. After the second time I applied tape to the underside to make it a really tight fit on the mount, so that it now requires several kg of force to remove it, and it hasn't come off since.
I might add that it has only ever come off my road bike, not my MTB, though I haven't used it a huge amount on an MTB. I think in many ways it gets a worse type of vibration on the road bike than the MTB. The road surface on one of the hills where it happened is truly dreadful, and someone else once told me that their 705 had on several occasions switched itself off at the top of the hill, which I suspect is also related to the pounding it gets from the road surface there.