My daughter keeps watching them over and over to the point where I'm so bored I'm making up theories about it to keep myself amused. But this one sounds pretty good I think.
The Who's in Horton are actually the future descendants of the Grinch and his Who's. The Grinch and Martha May eventually got together and had kids which lead to the Grinch's DNA being ingrained into the whole Who population which explains why the future Who's are all furry like the Grinch but still resemble the old Who's. When Jojo goes up to the old observatory we see its set on a hill/mountain which looks like the one in which the Grinch lived. The dot on which they lived was originally attached to the snowflake but when they reached a hotter climate the snow melted leaving the dot ending up resting on the flower. This change would have been catastrophe to the towns buildings so they had to build more sustainable structures and the observatory was built to try and research why this would have happened (by now the Grinch probably passed away leaving the place in need of a use); they never discovered any answers though as then after that the flower lay undisturbed so eventually over the years the whole thing became forgotten and the observatory abandoned leaving jojo to use it, then horton finds them and the plot of the movie happens.
What do you think?