As it happens, I threw a bag with tubes/tool, etc. on for the first time for a longer ride this past weekend. I didn't have the same experience you did, mine would slide off my back to my side and sway. Didn't like it a bit. It felt like it should have another strap to hold it on my back.

I used to ride all the time with a bag attached to the bike, but when I rebuilt the bike, I stripped off all non-essentials. Took one ride to fall in love with the responsiveness and acceleration without 5-7 lbs. of crap strapped on the bike.

I gotta take a look at how to do the messenger bag thing again. It gets old not going far for fear of flats or breakdowns.