I'm not sure that there is a phenomenon. 3 big mammals instead of 1 in one
spot may not be statistically significant and tell more about the quality of
the fossil record. For why there weren't any big placentals (and marsupials)
before -- how old is Eutheria in the first place? End-EK?
How big are *Deltatheridium* and the "Gurlin Tsav Skull" (a stagodontid),
both from before-end LK Mongolia?
What should have caused the relaxation in size constraints?
Perhaps more attention has been merely applied to the Lancian formation, which is why we have so many more big mammals. It seems that their would be available niches for larger size mammals throughout at least the latter half of the mesozoic.