They swapped the parameters (after a long period of either order being valid, then the old order being deprecated) to more closely match those of explode(). It was a bug fix if anything; the old form should have been more "offensive" than the change.
It broke programs for some sense of aesthetics. First rule of infrastructure is to not do that, like Linus' "Don't break userspace" command. By definition that can't be a bugfix.
