my DW opens always in that mode which I last worked in with a file and then closed DW. To make it certain I've tried it just in the moment.
That worked for me also till some time ago. Now it does not.
So I am looking where this can be setled.
I found out that you were right from the beggining. I found out, what was wrong:
I had three pages opened, but working on only one. The other two were opened in code mode. So, each time I started DW, every one of three pages opened in code mode. I fixed that and everything is as it should be.