How to shift the canvas? (rotate?)

[Krikor]

I don't know how I got it. 

I was with the free select tool selected and tracing a selection, using keys like ctrl, shift or alt, or a combination of them, I'm not sure what I did.
Then suddenly my canvas shifted a few degrees as shown in the image below.

I tried to understand what I had done, and I tried to recreate it, but to no avail.

In short, how could I have done this?