I'm not saying there is no price fixing, but this could also be explained by the supermarket chains all using the same suppliers (which is most definitely the case since the dairy market is not as diversivied as one might think; it's dominated by a few big players) and those suppliers already doing cut-throat price competition with razor sharp margins, meaning that if the slightest cost increase happens then it impacts all of them and once one can't take it anymore and raises by a few decimal percentage points, everybody else will immediately do that to.

Ever been to a gas station? That raised the price multiple times a day just like the other gas stations a few hundred yards away? Same principle. Doesn't need price fixing as explanation. They are just all buying the same raw product.