Comment 3 for bug 167210

Yes, this is Connector related. The
inkscape:connector-type="polyline" attribute marks the line
as being a connector, and moving it causes it to reroute.
If there are no obstables for it to avoid it will just
reroute to become a straight line.

As was already noted, removing the attribute results in the
expected behaviour.

I fell that it would be nice to have some sort of visual cue
that a particular selected line is a connector.
Double-clicking on a line will switch to the connector tool
if the line is a connector. Adding a visual cue and a way
to switch a line between a standard path and a connector are
on my todo list.