I think it's the same for all YouTubers, a few that I watch (non motovloggers) say that their views drop off at this time of year because of the good weather. And the college students (who would otherwise be studying by watching your videos) are out trying to get summer jobs etc.
Also, as motovloggers, we're also competing a lot harder against other motovloggers for people's free time as every motovlogger is out enjoying the weather and recording. Well, "competing" sounds a little stronger than I meant it, but I can't think of a synonym. It's different in winter when the men are separated from the big girls' blouses.