An excerpt from Wikipedia:
"The sangha traveled through the subcontinent, expounding the dharma. This continued throughout the year, except during the four months of the vassana rainy season when ascetics of all religions rarely traveled. One reason was that it was more difficult to do so without causing harm to animal life. At this time of year, the sangha would retreat to monasteries, public parks or forests, where people would come to them."
Even today, In India and perhaps in other countries where Buddhist/Jain monks reside; it is a practice amongst Monks to refrain from travelling during the four months of rainy season.
Assuming that it is not because of the difficulty in travelling but rather because they do not want to harm animal life:
and also knowing well that they allow people to come to them for guidance and a lot of people travel to them; is it not true that animal life is harmed even when a lot of people travel to them and Monks themselves are not travelling? From karmic viewpoint : What difference does it make if the animal life is being harmed by them or by the people who come to them?