Preview sketch tool underlines going from horizontal to vertical.

I have a macbook Pro that uses Yosemite - actually this problem started when I switched over from Snow Leopard to Yosemite. The problem is seen in the attached image.

I underline pdfs in preview using the sketch tool, but occasionally when i reopen these files my underlines go from horizontal to vertical and reposition themselves on the pdf as seen in the screenshot - why is that?