I have noticed quite a few times there are loose rocks on trailer beds. This is a violation of DOT regulations and can get you fined and go on your personal PSP DOT record. Also, if a rock comes off the trailer and hits a car, in some states it may be written up as an accident—placing an accident on your MVR, raising your personal car insurance, going on your DOT record, going on the company record, resulting in fines for you and the company—not to mention if it caused an accident and someone got hurt!!
Please watch as rocks may fall out of other cargo not just a rock load, also inspect a trailer that you pickup as someone else may have left rocks on it, but you will be responsible if you drive off with it.