Hi Milo,
I thought you might want to file this away in case it happens to someone else...
Tonight the front page of my nature theme site went single file... again. Given what I had to do to rectify last time I was sad.
I was just sitting down to work on it - and put up a post about technical woes first. I couldn't figure out why another post was at the top....
Then it hit me - it must be sticky.
When I took the sticky off - the whole format went back to normal.
I'd be curious to know if you can replicate?
Tania