Maybe the rotation is so slow that there is a significant differential between the rotation speed of the core and the surface, with layers of the upper mantle moving like an atmosphere (due to the differential and not just heat convection)? Then the surface rotation speed could change like the average windspeed in the upper atmosphere. Patterns or eddys in the motion could follow a cycle that speeds or slows the surface at regular intervals.
Or perhaps the absence of a moon means that the gravitational bulging due to the Sun is not countered, and results in greater friction and slowing of the planet's rotation relative to other planets?
Or the lack of a moon's disruption to the gravitational field allows small currents in the mantle to grow uninterrupted resulting in the effect I first mentioned?
Or could the slowdown be a countdown to another resurfacing event?