Another way to look at it is that the corner forms a three-sided pyramid-shaped conical horn having an expansion equivalent to a four-sided horn with 70o wall angles. It's sort of like a giant 70o CD horn. But below 35Hz, the wavelength is long enough that the entire height of the wall/corner junction comes into play, not just the expansion from the floor apex. At very low frequencies the launch corner isn't a three-sided pyramid but rather it is a four sided boundary having horizontal flare of 90o and parallel vertical sides. This is a different spatial orientation than the corner apex triangular pyramid, and has a different expansion rate. It acts like a tightly constrained space with increasing directivity as frequency drops.

To illustrate, see the following charts: