There are certain channels in the 5Ghz range that require the access point to do checking to make sure they don't actively interfere with radar...

See here for a document from Cisco describing the issue:

http://www.cisco.com/c/en/us/td/docs/routers/access/3200/sof...

interestingly enough, hard setting the channel to 100 might violate FCC regulations, and DFS should still be used...