It's most likely that the drone that crashed was similar in size to the type of bird that was spooked (Mavic Mini to Mavic Pro range; maybe a Phantom at most). Anyone flying a larger drone (Inspire and up) would have a serious license and familiarity with permits, therefore unlikely to be called "illegal".

I'm guessing it either fell after colliding with a bird or was flying low enough with sensors off and clipped the ground.