Hi,
I believe it could be due to speed/duplex mismatch on the interface:
"Interface has a TX buffer for outgoing traffic, when the rate of traffic pushed to this buffer is too high the interface cannot handle the amount of traffic. Interface starts to drop packets when the TX buffer is full and thus increments the underrun counter. This can be a sign that the interfaces are running at an inferior speed and/or duplex, or there is too much outgoing traffic that goes through the interface than the buffer can handle."
Cheers,
Ashvin