The Spanish aval (“guarantee, as in a bank guarantee”) comes from the French aval, meaning “downward”. The French word comes from the Latin vallem, meaning valley — a valley does slope downward, after all. From that same root, we get the English… avalanche, which is an overwhelming amount of the valley tumbling downwards!
But all this leaves the question: how did the word for “downward” turn into the word for a “loan guarantee”? That part is unknown. But we could speculate that the creditor calling upon a guarantor to pay in the case of a default is a low point for the borrower. Or perhaps, you need a guarantor only when you’re at a low point yourself. Or…? Since we don’t know the history, we can create infinite variations that sound like they might make sense, as a fun exercise.
You can see the a‑v-l root in all the variations clearly.