Here's kind of a funny theory I found on the Animorphs WMG page.
From TV Tropes:"Rachel unintentionally took Crayak's deal."If you ever find yourself desperate, Rachel. At an end. In need. Remember this: Your cousin's life is your passport to salvation in Crayak." Rachel killed Tom."
Oh, and I remembered another one.
9. The Helmacrons hacked the morphing cube. You know how, in the earlier books, it required an Andalite to be touching it for it to work? Even for David in #20, it didn't activate until Ax 'willed' him the morphing power, even though he touched it before that. But, later on in the series, everyone (and every
thing, if you count #39) who touched it suddenly got morphing powers. Loren, too, only had to pick it up in #49 to get powers from it. Note, though, that these occurrences were all post-#24, which is significant because in that book, the Helmacrons used the cube for what was clearly
not its intended purpose. My theory is that they had to hack into its programming somehow in order to use it, and in doing so they 'corrupted' it, permanently overriding the need for an Andalite to use that specific cube.