Coupled Monte Carlo neutronics and depletion problems have been noted to produce nonphysical power oscillations for large, spatially-decoupled problems. Previously these oscillations have been attributed to `numerical instability' - this work proposes that a prominent contributor to this phenomenon is neutron clustering during the Monte Carlo simulation, resulting in a poor estimate of the transport solution. This is demonstrated using insights from recent work on clustering and applying it to standard practice for Monte Carlo/depletion problems by performing simulations with the same number of histories - both total histories and only active histories - but different numbers of particles and cycles. The results demonstrate that neutron clustering appears to trigger instabilities in the problems considered and strongly affects Monte Carlo neutronics/depletion simulations.