The most common cause of this was the agent's phone restarted or a CTI interruption occured. This is typically something to start troubleshooting on UCM side. The CAD Troubleshooting Guide says essentially the same thing:

Problem The agent’s state changed to Not Ready for no apparent reason.
Symptom. In some situations, an agent’s state may change to Not
Ready for no apparent reason.
Cause. To determine the reason, check the reason code:
..other causes removed...
■ If the reason code is 32759, the agent’s state became Not Ready
because the phone went out of service. Check to make sure the
phone is still functional and that you can call the phone directly. If
everything seems fine, it is most likely a temporary problem and the
phone has since recovered. If the phone is still down, it is most
likely a Unified CM problem. Please consult Unified CM support.
Solution In many cases, an agent’s state becoming Not Ready is not a serious
issue. Simply click Ready to change the agent’s state to Ready.
To determine the reason code, do one of the following:

...other solutions removed...

If the reason code is 32759, the agent’s state became Not Ready
because the phone went out of service. Check to make sure the
phone is still functional and that you can call the phone directly. If
everything seems fine, it is most likely a temporary problem and the
phone has since recovered. If the phone is still down, it is most
likely a Unified CM problem. Please consult Unified CM support.