when using my tape measure, normally one can make parallel guidelines. Every now and then though, it just makes perpendicular lines, which is a total mystery to me. On the other hand, sometimes I need to make a perpendicular guide line and then I don’t know how. Did I miss something in my basic training?
I just placed dozens of guidelines, but could not reproduce your issue.
Do you use your ‘arrow keys’ to ‘toggle lock inference direction’ as shown at the bottom info bar?
Not sure if this would prevent your guidelines to rotate 90 degrees, but it might be worth a try?
The tape measure has to have something to make the guideline parallel TO. That’s usually an edge, but can be an axis. If you are on the midpoint of an edge, it will make the guide parallel to the edge.
It you start from an endpoint, the centre of a circle, or an intersection, it will draw a guidepoint.
But I’ve never known it draw a guideline at 90° to the starting edge in normal use.
However, like AnonNiem above, i found that if you accidentally touch one of the arrow keys during the mouse move to drag out the guideline, the guideline will draw parallel to the axis corresponding to the key you touch - Red (x) for Right arrow, Green for Left (y), or Blue (z) for Up arrow.
That might explain what’s happening, but I’ve never know it happen accidentally for me.
With the guide tool active, double click on the surface
it still doesn’t explain why I have perpendicular guides. And I can’t find any toggle to change that. I use the toggles a lot.
I have used the guides a lot, but haven’t had the issue since recently. and only on a few spots.
Just thinking out loud, could there be a tiny, almost hidden, line at the point you want to place the guideline which is perpendicular to the line you want to offset? 
Not sure if it is a problem of the software of something in your model, maybe you could share the model here?
Meanwhile I have erased those naughty guides, so I can’t check that out. Don’t think it’s some point or so.
The only thing I can think of is that you are using the Protractor Tool for perpendicular guide creation…
I think I had a similar problem. I try to create a parallel guide but instead I get a perpendicular dotted line that terminates in a plus symbol. Also, when you hover the tape measure tool over an edge, you get strange information like “intersection in component 244” followed by three sets of measurements. Also, the measure tool snaps to an edge and shows an X, not the usual square.
Turns out I had edges turned off. View > Edge Style > Edges unchecked. Once I check this again, the measure tool functions as I remembered it always did. Hover over an edge and you will get its length. Click on an edge and you can create a parellel guide.
Not sure why the differing behavior, but just be careful when you hide edges. The tape measure tool becomes a different animal.
For me SketchUp seems to be performing as expected.
You may have been dealing with “the beast’s short time memory”. I don’t know what SketchUp steps you did prior to hiding edges (in component) and / or disabling edges (in entire model).
It’s handy if you have setup shortcut keys for:
- ‘View Profiles’ , for me: [Alt] + [e]
- ‘View Edges’ , for me: [Shift] + [e]
(these are Windows combinations!)