There is clearly a duty for the airline to properly train staff and to ensure that its staff have the necessary tools to provide accessibility in transportation.
In my view, that duty also extends to any subcontractor the airline hires, as agents or in any other capacity, to do work for the boarding and disembarking of passengers on that flight.
Part of the problem is perhaps a cultural issue. There may be a culture of cutting corners, of “let's not do things by the book”. What needs to be instilled in them is that these things do matter, and that is why significant fines would be important to get the point across that this is no joke. This is a serious matter.