If you have already watched the finale of Star vs. the Forces of Evil ("Cleaved"), I get that there was a portal which resulted in the cleavage of Earth and Mewni, but what caused that portal to open?
1 Answer
My theory is it's the fallout from Omnitraxus Prime's Death. Without him to regulate space-time the dimensions merged together.
Alternatively Marco and Star just belong together so much that magic merged together the different dimensions just to ensure they got sent back "to where they belong".