a knee-jerk reaction; to do/react to something impulsively/naively/without thinking much
It is often used ironically when someone always responds to someone/something (e.g. web news) in a naive or predictable way (see the example sentence below). It is also used when you do something impulsively, e.g. buy something impulsively when you see its ad on the internet.
Whenever celebrities do donation, some people naively criticise them, saying that their ulterior motive is to make themselves famous.