Time Machine must create a new backup for you

I get this message (screenshot below) every few weeks now. Not sure why.

Here are my specs (screenshot):

Originally I had a 2Tb usb harddrive connected to my netgear nighthawk r7000p router and was using time machine on my mac for backups. Was this way for 2 years. Never had issues with it for the longest time and then started getting a message a few months ago from TimeMachine saying that i needed to create a new backup. Seemed to happen every few weeks. So last week I remove that usb harddrive connected to my router and installed a Buffalo Terastation NAS on which I created a successful TM backup. Incremental backups have been successfully backing up to it ever since. Then this morning i get this message again that I need to create a new backup. I know I can scrap the network backups and use a connected USB hdd but I dont see why I keep having this problem with my network backups.

Any insight would be helpful.

Thanks.