Around February there's a whole week where schools are closed with the incentive to do sports - especially winter sports (given the time of the year). A lot of people go to ski resorts - the alps (or up north in Sweden - which got mountains but is far from the Alps).
I lived in a town of about 800 people during the onset and first year of the pandemic. More than a dozen people from my town alone were in Lombardy and a couple of the super early outbreak areas of Germany during 2020 sportlov (the break that stpe mentions in parent comment). I knew first-hand a lot of people who were really ill in March and April, though I don't know if it was covid for sure, since our testing was insufficient then. Anyway, yeah, lots of Swedes traveling to the wrong places at the wrong time right then.