Replacing the NIC by itself shouldn't trigger a reactivation but it comes close to the threshold. The problem is one of the pieces of system identification information is the MAC from the NIC.

Did you perhaps do something else in the last 4 months? You could have had another change that was below the threshold and then added the NIC on top of it and put it over the limit. Various changes have various point scores, too many points and you need to reactivate.

Note, also, that many people with apparently legitimate copies of Windows have had trouble because the manufacturer did something they shouldn't have. Sometimes innocently (they had the licenses, they just copied the HD image), sometimes not so innocently.