For the last year a bot of some kind has loaded Fallback Directory relays with excessive directory document requests. Some detail can be found in ticket #27921 at https://trac.torproject.org/projects/tor/ticket/27921.
To determine if your relay is affected, look the relay up in https://metrics.torproject.org/rs.html, click the 6-Month history tab, and examine the graph for significantly higher "write bytes per second" relative to "read bytes per second".
I find this annoying (since I pay for the wasted bandwidth) and came up with a one-line hack that mitigates the abuse without impairing the usefulness of the DIR service to normal relays and clients. If you compile your daemon from source and are interested contact me directly for the patch.