Maybe she knew that when her son gets to that stage it triggers off a different behaviour..

I know with both mine, what seems an innocent running and squealing is about to turn into extreme meltdown mode for no other reason than they have just psuhed themselves that little too far. Only now is my DH and my Dad who also live with us are starting to understand the particular moment.. They kept telling me I was over-reacting, until a few times of late, I let them take control and meltdown mode kicked in..

I am all for children running around and making a noise in a playground, but also know the limits of my kids and I could be the mother you descibed above. (not meaning I take offence, but giving a scenario that it could've been)..