Why do people do this!!!!!

[Young rider]

Well if it has been blown over then maybe it'd be worth putting the bike on the side stand but facing in to the direction of the wind.
It may look a bit out of place but if it prevents it happening again then it'll be worth it.

Or is that concrete a large step? I can't tell for sure from the photo'?

It's a concrete step I'm just gonna set up a cctv then we will know who has done it.