Router safety - what did I do wrong?
If the router bit was in the collet the correct amount and tightened correctly then you should have no issues. I noticed a few things when looking at your picture, it is an older shank that had some rust on it and a few dings, it is possible it is not seating correctly in the collet.

With that said there is the possibility that the shank walked out of the collect because there was to much pressure in a climb cutting position. In other words the shank was rotating clockwise and you were moving left to right, the router bit is walking over the piece and every once in a while it grabs and pulls the shank out of the collet slightly like a screw. It DOES NOT matter how tightly you lock it in the collect. 

Working in the machine shop, milling machine work the same way, I have see cutters come flying out for the same reason. It happens. Best advise is to work right to left and avoid climbing on the piece, finish will also be better. If you must climb take smaller cuts.