"Panic" is an anthropomorphism for what is really going on. Plants react to water. That makes complete sense. Humans react to it when it falls on us as well. Maybe we go inside. Maybe we open an umbrella. Maybe we go dance through the fields and enjoy a summer rain. But whatever we do, they are reactions, not "Panic."
The word "panic" is in quotes, as a simplification of "invoke a defensive and protective reaction", and to get non-scientists to read a science article... not a terrible thing.
Edit: the original article does not quote "panic", but Professor Harvey Millar uses the word, so not clickbait?