The period you mention is peak travel time in Europe this year (but wasn't peak travel time last year).
Ascension Day and the Monday after Pentecost are public holiday in most of Europe. They are ten days apart, and a lot of Europeans use the Ascension/Pentecost period for travel. Last year Ascension was at the very end of May and Pentecost was in June. This year they are on 5 and 16 May respectively.