Is there anything connected to that port ? sounds like you have more than one device on there, or you have a phone and a pc, but the phone is not getting put into the voice vlan perhaps. What does the switch log say ?
Can you confirm CDP is enabled at the port level and that with dot1x config the phone is assigned to the voice VLAN? (i.e check it without the PC connected to the phone so the port doesn't err-disable)
I'm also wondering what AuthC method you have setup that the PC is authenticating via MAB and not dot1x.