We had rummynose tetras do this in the shop. It was caused by newly imported fish not getting enough time to adjust to different water conditions. They would be kept in really acid water in Asia and put into slightly acid water here but they would either die straight away, or be really nervous and skittish for the next few weeks. If we did a water change within 2 weeks of them coming in, they all went belly up.
We used to hold these fish for about a month before selling them because of this issue.
A slight change in water chemistry (pH, GH or KH) can be enough to kill them.
If the fish had only come in recently, the transport stress is enough to kill them.
If the shop assistant chased the fish around the tank, it can kill them.
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Contact the shop and let them know what is going on. Maybe take the bodies and a sample of tank water to the shop so they can test it.
If you do take fish and a sample of water in, take them in separate containers so the dead fish don't affect the water quality.
Compare GH, pH and KH with the shop tank.
Keep pictures and records of any more deaths.