I think everyone who isn't used to helicopters tends to underestimate their downwash, full stop.
Quite recently I watched a pilot park a PA-28 directly adjacent to us, already parked on the helipad, in a 3 tonne helicopter, ignoring the other parking spots further away in the same row. The pilot was about to walk away, also ignoring the tie-downs nearby and her aircraft control locks. The only reason I can imagine she parked there was that it was the closest parking spot to the clubhouse.
She seemed quite disgruntled when I walked over and politely advised her that it might be a good idea to at least put some tie downs on her aircraft's near wing, as we couldn't ground taxi and we were about to lift.