Rainboe
The biggest hazard for towbars has to be new push back drivers learning to be gentle starting and stopping the push, or trying to change gear when towing. Over time fatigue builds up in the shear pins and eventually one or more say enough is enough. The pins are only normally replaced when they break, rather than after a certain length of time so the only real check they are ok is a visual before pushing or towing.
Of course pushing without a bypass pin installed (on those a/c that need one) has been know to shorten their lives a little as well
