I get the same, and think it
I get the same, and think it is a nutrition issue. I’ve found that avoiding the things I really fancy when I get back helps. By that I mean anything sugary; cakes, biscuits and sweets etc and definitely espresso based coffee (my favourite!).
I always want a double espresso shortly after I come back, and always end up with a major headache for the rest of the day.
I now have something like sardines on toast and orange juice, and avoid sugar and caffeine for a couple of hours after a long ride. This has made quite a difference and the headaches are much rarer now.