The simultaneous occurrence of wildfire can hinder firefighting effectiveness via multiple mechanisms that might explain historical resource demand trends. We validate this hypothesis by using data from the Monitoring Trends in Burn Severity (MTBS) project to determine if simultaneous wildfire occurrence is correlated with preparedness levels and examine potential changes in simultaneous wildfire activity over the 1984-2015 record. We explore patterns that are helpful for predicting simultaneous wildfire, such as seasonal variability in simultaneity, cross-regional correlations and models of simultaneous wildfire occurrence based on dryness and lightning indicators. We show that simultaneous wildfire is at least as correlated with preparedness levels as other burned area measures and identify changes in simultaneous wildfire occurrence within the western and southern United States. Seasonal variation and spatial autocorrelation in simultaneous wildfire occurrence provide evidence of coupling of wildfire activity in portions of the western United States. Best-approximating models of simultaneity suggest that high levels of simultaneous wildfire often coincided with low fuel moisture and high levels of lightning occurrence. Model uncertainty was high in some contexts but, with only a few exceptions, there was strong evidence that the best model should include both a dryness and lightning indicator.