I had similar concerns when I first got my Turbflotor 1000 Multi. This is what I've found... do tilt it in about 5 degrees by using the adjustor screws at the bottom of the skimmer. Should you remove the outflow boxes, it will then not fall (the instructions actually touch on this a bit).
Secondly, make sure you tighten the screws on the outflow boxes themselves. The first box will really prevent the skimmer from falling.
I've knocked my around quite a bit and the box has prevented any harm on several occasions.
On the times that I had the box removed and knocked the skimmer (yes, I am clumsy

- and a 10 gallon sump doesn't help.) the adjustment screws at the bottom of the skimmer have prevented a major mess by leaning the skimmer in so gravity takes hold in our favor. However, it will slip a bit on the outflow pipes in these cases and one time it was violent enough to chip the pipe a bit. So be careful with the box off.
If you don't use the boxes because... well they are huge (but they do work well at decreasing bubbles in the tank) .. keep this stuff in mind and make something to secure the skimmer.
I agree with Raven here about this skimmer - I love mine and highly recommend them - but to keep costs down, they didn't use the really nice and thick plastic like EuroReef, so you have to baby it a little bit when moving stuff around. That said, mine has taken some serious knocks and kicks out the skimmate with minimal adjustments. Hope this helps.
matt