The short answer is, we don’t know. The likely answer is that it is normal. In nature, population fluctuations happen. There is a fairly long time-series of spruce budworm trap data from the pheromone traps and an even longer one from light traps. In those data, we can see that the year-to-year catches do not make a smooth trend line when plotted over time. Small fluctuations may be even more apparent in times when budworm populations are low. We do have reports of other jurisdictions with similar results in 2016 despite using a different pheromone supplier, so the rumor that the pheromone did not work is not likely. 
Light trap catches were up compared to previous years—a look at the dates moths were caught reveals that many of those moths were trapped during the same time there was a large-scale migration event from the defoliated area in Quebec. Canadian researchers looked at sex ratio from those flights, and found that 80% of the collected moths were female—those would not be attracted to pheromone traps, but may be lured by light traps.
For more information, see the 2016 Spruce Budworm Report from the Maine Forest Service.