Activation is based on the MAc Address for the active network card. If the mac address changes for sdome reason, it will cause the need for reactivation
a couple of common scenarios are:
1) Changing the connection type. For example if you connect using a hard wired connection, then change to a wireless connection it will cause the need to reactivate.
2) VPN Software.
some VPN software will create a virtual network adapter that changes the MAC address.