Does volcanic activity fluctuate?
This is probably an observational effect that is quite common in the Earth Sciences. In scientifically progressing societies there is a higher proportion of observations due to a number of effects:


awareness of science (not interpreting it as a wonder)
ability to record events
ability to observe (think 12 hour day in the factory vs. free weekend)
technology to do remote observations
...


This is also why more and more meteorites are observed falling. In the middle ages you were either too busy to care, took it as a wonder, or found a meteorite, but had no ability to write it down. Similar scenario for volcanic eruptions. I could imagine that actually all those ships, airplanes, and oudoor-activities (no offense) lead to more observations of eruptions, but that people were to busy recording and writing them down. The geologists of their time were probably busy finding war-necessary resources.

Sources:


http://www.volcanodiscovery.com/de/volcanoes/faq/is-volcanic-activity-increasing.html