Fred MacMurray's bobble headed talking

Anyone else notice that nearly every time Steve Douglas talks, he bobbles his head quite actively at the neck? It seems like some peculiar nervous habit, but I don't recall noticing it in other characters he played on film. Maybe it was by instruction of the director for My Three Sons, but as an adult watching it all these years after I saw it as a child, I find it disconcerting to watch him talking to his sons. The verbal message is pretty much right there on target, but that wiggling head is weird.