That would be due to a BIG-IP monitor not getting a successful response back from the servers within the timeout period. You can check the /var/log/ltm log file for additional info. If you need more detail on the monitoring requests and responses, you can enable debug logging for the monitoring daemon, bigd, using:
b db Bigd.Debug "enable"
By default, the logging is written to /var/log/bigdlog.
You may find that your interval and timeout aren't set optimally. Normally, the interval should be greater than 5 seconds. The timeout should be set to 3 x interval + 1 second. The default interval / timeout are 5 / 31.
Aaron