Why do low concentrations of $rm H_2$ in $rm He$ gas exhibit lower thermal conductivity than either?

In the 1960's, it was thought that anomalous cross-relaxation was required to explain the minimum. However, just a few weeks after you posted this question, a more convincing partial answer was published in Physical Review E. According to the paper, the observed minimum can be modeled accurately in the context of revised Enskog theory using a simple binary hard-sphere model.

I'm not sure if there is a good intuitive explanation for this behavior; the paper depends heavily on computational modeling. If it's helpful, the important unusual feature of the hydrogen-helium system is that hydrogen molecules are much lighter than helium atoms (2 vs. 4 amu) but are physically larger. The paper predicts that other systems with the same property, such as krypton/nitrogen, may exhibit similar behavior.