Weak building materials

Yesterday I extended my desk in my computer-room.
Since I don't want the desk to be on the floor, because I'll trip over it, I want to fix it to the wall.
So I bought these carriers, which should be able to uphold about 80kg each.
After mounting them, adjusting the desktop-plate to fit and rearranging some cables under my desk, I tried to get up from the ground.
So I lifted myself from my knees using only 1 hand, leaning on my desk.
Which result this diagonal rod to fold and me falling on the ground.

I fitted a (hopefully a temporal) supporting beam which can be seen on the right.