This is strange and not sure if its directly related to DisplayLink, as we only provide the video part of the product, so keyboard presses should bypass our chip and driver.
This could be checked by uninstalling DisplayLink and see if the same problem persists. I'd try swapping the keyboard and USB ports to see if it solves the issue.
It may also be that they are typing and the screen is not refreshing until they do something, which would be a different problem. I'd check its not due to some strange screen corruption, which can be worked around by following this thread:
http://www.displaylink.org/forum/showthread.php?t=897
Wim