Volcanic activity is unpredictable and well is explained by Murphy's Laws. While providing comic relief, it is also hoped that these rules will aid serious safety planning.
Nothing is as easy as it looks.
Everything takes longer that you think. Left to themselves, things tend to go from bad to worse. Every solution breeds new problems. If one volcano is about to erupt you will either get there one day too late or leave one day too early. A decrease in eruptive activity increases the risk.