Hear me out, for the "Undertale Blue falls into the underground first before Yellow falls after to find out what happened to him" scenario...
What if Blue told him that he has a special surprise for him in the forest near Mt. Ebott because Yellow continued to encourage him to keep dancing and doing ballet despite being bullied for it that Blue wanted to give Yellow his own special ballet performance?
But Yellow wasn't able to come despite promising that "Of course he would come! They're friends after all!" since his parents needed his help and it reached until nighttime, way past the time they agreed to meet in the forest.
Thinking Blue just got stood up by Yellow, he thought himself foolish to have thought another boy would've actually enjoyed his passion for dancing arts and decided to wander around the forest instead to clear his mind since the flowers and butterflies would at least be willing to lend their eyes to his sorrows
(Besides, he's going to have to explain himself to his parents where he went off to and why he "borrowed" his mother's ballet shoes and tutu temporarily once he'll come back home. Some time spent sulking and gathering himself up before coming back home will be needed for that)
And well... it was then that he accidentally fell into the underground. Be it his ignorance, carelessness, or lack of thought for just a moment, his fate is sealed.
