A few odds and sods. First, make sure that you have the silicon cover the right way around (chamfered edge towards the mount, 90' edge outwards) - the interior of the cover has some ridges that press against the open door etc. to keep them in place. Next, make sure that the cover is pressed firmly into place - it should sort of 'click' (well, as much as silicon can) into the normal catch used to close the door, but not too far - it's always possible that you put so much pressure on the interior that it all rebounds on you.
Finally, if the pattern is the same as mine, you'll have two hold and a slot at the back - before mounting the camera, make sure that the supportive area around the cable as it enters the plug only comes flush to the hole itself - any further out, and the plug is not in the socket correctly. If it is, press it in so that none of the plug is trying to press its way out of the holes. As long as you do that before setting off, the flexibility of the silicon cover should keep it in place during the journey.
I'm guessing that it was not correctly in place before you set off.