The Curve.Extrude (distance) node uses the coordinate system of the curve to pick the direction which it extrudes the curve. When modeling/drawing symmetrical objects things tend to be mirrored, which inverts the coordinate system (hence the downward extrusions). Similar issues can come about when using fillet and other ‘shortcut’ methods which make drafting much faster.
To resolve this, use a different extrusion method - Curve.Extrude (Direction and Distance).
This screenshot should provide more clarity:
The by distance version is on the left side of the image, and the by direction/distance is on the right.