bike transport in bed of truck

Check the stem before every ride?

Is the bike bouncing around or swinging in the bed? I'm having trouble reconciling the truck mount and the bike problem as being related issues?

the front fork doesnt bounce around a bit.  But should i make sure the rear tire is secured as well do avoid it bouncing on the bumps? Maybe thats my issue.

As strange as this may sound to some it could be the vibration that the bike is experiencing in the bed of the truck loosening the bolts. I work part time for a company that ships products by Truck and for a while we were experiencing problems with nuts loosening during shipment. This only occurred in certain trucks, driving in a minivan or a car with the product equal or greater distances did not result in this. A solution may be lock tight if is can safely be used on your bike. I’m just getting into  bike maintenance yet so I don’t know if this is a safe and viable solution for you or not.

I just lay my bike down in the bed of the truck with the tires up against the back of the cab.  Is this a bad idea?  I haven't had any problems so far.

I often lay my bike down on several old pillows in the bed of my truck. This allows me to close the tonneau cover and keep it dry when it rains. So far, so good.

If you do lay the bike down. Remember to keep the drive train side up.