I have used GIMP for ages, and I've NEVER seen this before...
When I try to make a freehand line, it comes out all jaggy, since somehow horizontal/vertical/45 degree lines are preferred apparently:
What is this and how can I get it deactivated?
Your brush is snapping to the image grid. Reset with View > Snap to grid
On my system, with "snap to grid" enabled (left) and disabled (right):