As a non-native Japanese speaker, I would assume the following: That the sentence parses as follows
- subject (81 year old man)
- object (wheelchiar his wife was riding in)
- te form to indicate simultaneous action (while pushing)
in an adjectival sentence, ga can be conjugated to no in order to indicate that the subject is part of the adjectival sentence and not the larger sentence. in this case the verb oshite indicates simultaneous action. It therefore translates to:
There was an accident where an 81 year old man fell backwards after pushing his wife up an escalator in a wheelchair.