Millimeter wave communication (mmWave) allows high-speed access to the radio channel. Given the highly-directional nature of mmWave, dense deployments can be implemented with a macro base station serving many micro base stations, rather than connecting micro base stations directly to the core network as in legacy cellular systems. Moreover, micro base stations may cooperate in relaying packets to other micro base stations. Relays and spatial reuse speed up communication, but increase the complexity of scheduling. In this work, we study the mmWave wireless backhaul scheduling problem in the described architecture, assuming stochastic arrival of packets at the macro base station to be delivered to micro base stations. We present various results concerning system stability, defined as a bounded expected queue sizes of macro base station and micro base stations, under different patterns of random traffic. In particular, that almost all admissible arrival patterns could be handled by some universally stable algorithms, while non-admissible arrival patterns do not allow stability for any algorithm.