I wonder if bike fit, or your body getting used to your riding position after a long time off the bike, could be a factor? I know that strain/tension in certain muscle groups in your neck can cause headaches.
That said, I used to get next-day headaches after long backcountry skiing days. I think it was a combination of electrolytes and sun exposure/sunburn (compounded by altitude). They more or less went away when I began drinking a 1:1 ratio of sports drink to water and applying sunscreen way more frequently.