Hello peeps

I started a batch of alcoholic water the other day (1kg sugar, water &
yeast (Yes I could put some lemon juice in it so it has citric acid, but
I've found it does well enough on its own)). I pitched the yeast before
I went to bed, it was a cooler night than it has been recently so it
didn't surprise me that in the morning it hadn't started fermenting.

What did surprise me, is that the airlock had just started to pull air
into the demijohn. The water in the airlock was actually being pulled
into the demijohn instead of expelling gas like it should. I left the
airlock out for a few minutes (I figured that if the yeast wants air,
its wise to let it have what it wants). I put the airlock back in and
went for a shower. When I got back it had started fermenting normally.

Anybody else have this happen? Any ideas what caused it? The lovely
assistant at my local homebrew store was as surprised as I was at this.