  Understanding the sources of losses and chemical reactions of ultracold
alkali-metal molecules is among the critical elements needed for their
application in precision measurements and quantum technologies. Recent
experiments with nonreactive systems have reported unexpectedly large loss
rates, posing a challenge for theoretical explanation. Here, we examine the
dynamics of intermediate four-atom complexes formed in bimolecular collisions.
We calculate the nuclear spin--rotation, spin--spin, and quadrupole coupling
constants for bialkali tetramers using ab intio quantum-chemical methods. We
show that the nuclear spin--spin and quadrupole couplings are strong enough to
couple different rotational manifolds to increase the density of states and
lifetimes of the collision complexes, which is consistent with experimental
results. We propose further experiments to confirm our predictions.
