Looks like it is enabled by default (for at least some NICs) in linux when the receive buffer is too full.

https://www.kernel.org/doc/Documentation/networking/ixgbe.tx...

I wonder if it is an issue with the kernel not keeping up, or if it is due to some other odd hardware effect (sometimes "gigabit" nics on embedded boards are really connected to a USB2.0 bus, which is slower than gigabit).