Love the science of your view. Don't worry about why it occurs as long as you can use it to further your denier claims.Beats me.....
Simple answer is changes in the ocean current patterns (such as NAO).
https://www.newscientist.com/article/dn16892-natural-mechanism-for-medieval-warming-discovered/
And if you cared to look at it, it seems that the MWP may have been a LOCAL phenomena as climactic records from other parts of the world show a cooling during that period. What we have now in an increase in GLOBAL temperatures.