Congestion packets dropped on any switch are only packets dropped inside switch due to buffer being overrun on a physical interface. More than likely you are having a micro burst that is overruning the buffers... Link to how to look for this
https://extremeportal.force.com/ExtrArticleDetail?n=000010163
another link to steps to take for packet lose
https://gtacknowledge.extremenetworks.com/articles/Solution/Prevent-packet-drops
Again congestion drops on in the switch fabric when the buffers get overrun... Only Egress traffic on port and these switches all forward traffic at wirespeed so somehow some where there is more traffic in the switch than the interface speed can send to the adjacent switch / router. I did read a couple of post where flow control when enable can have adverse effects if the device you are sending traffic to keeps telling the switch to slow down a with a pause frame
Lots of other info in our knowledge base if you just search for port congestion.