If it timed out, that would happen.
Try this command to see the status of your radius servers. An overloaded Radius server would do that.
(Aruba-M3.arubanetworks.com) #show aaa authentication-server radius statistics
RADIUS Server Statistics
------------------------
Statistics Radius1 Radius-Server02 Radius-Server-03a Radius-Server04 Radsrv
---------- ------- -------------- ----------------- --------------- ------
Accounting Requests 0 0 0 0 0
Raw Requests 0 0 0 0 0
PAP Requests 0 0 0 0 0
CHAP Requests 0 0 0 0 0
MS-CHAP Requests 0 0 0 0 0
MS-CHAPv2 Requests 0 486 0 0 0
Mismatch Response 0 0 0 0 0
Bad Authenticator 0 0 0 0 0
Access-Accept 0 455 0 0 0
Access-Reject 0 31 23 0 0
Accounting-Response 0 0 0 0 0
Access-Challenge 0 0 0 0 0
Unknown Response code 0 0 0 0 0
Timeouts 0 0 0 20 0
AvgRespTime (ms) 0 8 0 0 0
Uptime (d:h:m) 7:16:42 7:16:42 7:16:42 7:16:42 7:16:42
SEQ first/last/free 1/255/255 232/231/255 1/255/255 1/255/255 1/255/255