These incidents happen all the time, and United is sued constantly for such things, all over the country, just like every airline. The difference now is the media is more willing to run with stories dropped by plaintiff's attorneys seeking publicity for their erstwhile relatively low-value cases because it fits the "UNITED HATES [fill in the blank... the disabled, the elderly, dogs, guitars, rabbits, pajamas, etc....] narrative.

Wheelchair cases are an especially common personal injury lawsuit against airlines for a number of reasons. First, wheelchair vendors are typically staffed by low-paid workers with limited training and high rates of turnover. Second, passengers requiring wheelchair assistance are often already in a compromised state such that they may be more susceptible to injury than the average traveler. Third, the airport/jetbridge/aircraft is a challenging environment for passengers with mobility concerns and the risk for mishap increases accordingly. I don't say this in an attempt to absolve anyone of responsibility for doing their job or taking care of paying passengers, but rather to describe the situation as it actually plays out, in practice.

United gets the top-line attention here, but in reality, the hammer comes down on the wheelchair vendor, usually on the basis of contractual liability-shifting provisions.