Aquariums: Help? Guppies suddenly dogpiling

Hi,

Posting in a small panic, just got home to my guppies suddenly dogpiling into one corner of the tank and I can’t figure out why.

I did a dip stick which showed nitrates and nitrites testing fine, and nothing else out of the ordinary.

Did a water change and added a sponge filter in addition to the tank’s hang on back filter in case it was lack of oxygen but even a couple hours later there’s no change in behaviour.

Any advice or ideas would be much appreciated