I would say it's probably because the weather is getting a bit warmer and they realise it's that time of year

I keep fancy goldfish indoors and they sometimes drive me mad at night times, thrashing around the tank and wrecking the place - they are kept at 22 degrees, as they don't do well in cold temps, but this means they constantly think it's springtime

I would post on here, if you have any questions www.fishkeeping.co.uk There are a few pond experts on there - very helpful