Chances are that you managed to hit Ctrl-L at some point.
I was thinking the same thing, but I guess the same as the OP that it's weird that it just recently started happening so frequently. Possibly in the v6.31 update, something was changed that made the line marker behave this way. Similar with
this report (same line of thinking, not similar issue), I thought it was something I was doing differently, but it wasn't and was fixed in v6.31.
