Nether portals that are linked will occasionally delink and create a new portal.

Eg: 7, 7 in the nether connects to 0, 0 in the overworld, and there are no other portals. It works fine 30 times in a row. Suddenly, when coming back to the overworld, a new portal is created at 56, 56. When you go back in and come back out, it takes you to 0, 0 as normal. But now occasionally It'll take you to 56, 56. If you break the portal at 56, 56, you will consistently go to 0, 0, until one day, it generates one near 56, 56 again.