If you look at the part that gets warped, it happens when the lens is at full extension - close up. It also happens because the moving part - that contains the glass and heavy parts - is being extended by the part that gets warped - what Canon calls the helical or helix (its really not, but they call it that). If the lens is "out" and you whack the front a bit, the parts that ride in the slots on the helix will snap out of place, or the helix itself will bend. There's not much holding things at full extension.
Moral - using the hood - which attaches to the non moving part, helps, it takes abuse off the inner moving part of the lens. Also - whenever you can - remember to put the lens back at infinity before storage.
Turning the hood around during storage... wont protect the lens.
Its not a great design... it could have been done better, beefier, but it is what it is.