The tubes of yogurt have a shell of thin plastic, with a single lengthwise seam and sealed at each end symmetrically apart from a small cut to initiate tearing at one end. They are of internal length 15 cm (diameter ~2cm). I assume that the microwaves used by the oven are of frequency 2.45 GHz, or 12.25 cm in wavelength.
I have noticed standing-wave heating effects in food containing structures with linear dimension of the order of 5–20 cm; but that has always been in a complex mass. In this case, the configuration is simple: a single horizontal, roughly elliptical, cylinder filled with a homogeneous product. As usual, the sample sits on a rotating plate to receive even radiation.
The tube gets too hot to hold at one end while the other end remains frozen, consistently after several trials with near-identical tubes.
I would not be too bothered if the heating were in the middle, or at each end, or even at two symmetrically placed nodes. What puzzles me is this: Why just one end? How do the microwaves tell one end from the other?