There are so many possible factors in such a change that the "why" can be hard to pin down. Certainly, different lots of powder or primers can make a difference, and there's not much we can do about that other than develop a new load. But my first suspicion would be a fouled barrel. You can think you are getting a bore clean, and it can look squeaky-clean, but stilll not be clean.
I well remember a .30-06 custom Mauser I had that was a tackdriver with its favorite load for years. When the accuracy went off, I nearly went nuts trying to figure out the problem. Then, a friend got a Foul Out unit and in desperation I tried it out (even though I KNEW that bore was clean!). The amount of copper fouling that thing removed was amazing, and the accuracy came right back overnight.