Horizontals are usually pretty stable. But I saw an 80 gallon vertical get pulled over by the air hose when someone was trying to make it reach just a little farther than it would.
If its plumbed into a hard line, thats a little better. But this one just had a hose hooked to it.
It wasn't mine, and I did not find out exactly what happened to it. But they said it snapped the outlet hookup off, blew up against the wall, and before anyone could get to it to turn it off, it hydro-locked from the oil running up into the cylinders and threw a rod.
Don't know if that was the cause for sure, but they had a new one when I went back.
If you cannot attach it to the floor, how about some flexible lines from the wall or ceiling to keep it from falling over?