Hi Fred,
I believe this network impact was caused by a loop. Maybe right after adding the ports to a vlan.
Depending on your network topology you have some loop prevention mechanisms.
You can use STP/EAPS/ERPS for ring topologies which will also keep one link blocked to prevent looping and at same time provide redundancy.
Another option would be run ELRP or even track MAC.Moves to try to find which device is causing the loop.