Trimet Director of Transportation responds to my request for clarification

Al,
Sorry for my slow response.

The relevant SOP is B804,Leaving a Bus Stop,

6. Turn your head left for a second scan of the left side and mirror. Scan the interior center mirror as you sweep your vision back to the left to check that your customers appear ready for the bus to move. Look over your left shoulder and rock and roll (lean) in your seat as you scan left to increase what you see in the mirrors, and to see around vision barriers.

What I understand this to mean is this:

It is not our policy that the bus cannot be moved if someone is standing instead of sitting.
We ask operators to observe whether a person "appears ready for the bus to move," that is, appears steady and will not fall if the bus is moved.
Thus senior citizens, disabled citizens, or customers not holding on to a stanchion would be passengers that might cause an operator not to move until they were seated or clearly "stable"

If there is a fall before someone is seated this would not result in an automatic PA because such factors as mentioned above would need to be reviewed to determine if the operator was using good judgment.  A reckless maneuver might be an operator pulling away from the curb at a sharp angle while a senior citizen with a cane is still walking down the aisle towards a seat and not hanging on to a stanchion.