I don't recall ever seeing anything like it, ever, and i've been watching for decades.
Knees, yes, covering the face, sure. But just gently sitting back ? lol.
Not that it's wrong - it's perfectly fine, but it just looked so awkward !
She was there so long that she had to run to Serena who was already on Maria's side of the court for the handshake !
p.s. as tasteless as Maria's yelps at SW df's were - she did seem to say something nice to Serena at the end about her injury. Serena looked as if to say 'it's ok, good match' or something.