i am not an expert on global agricultural markets, but my suspicion is drought, followed by a global (human) pandemic, but i don't know if those actually caused it even if you could prove they (both) happened. you can also see a significant drop in the 90s correlating with mad cow disease. there it's easy to say "we destroyed a bunch of cattle instead of slaughtering them" but that's not exactly reducing suffering. i seem to recall similar stories during the pandemic.
i highly doubt we could draw a causal link between buying beans and either of those dips, though.
I would choose one that is provably effective. actually rescuing animals tangibly saves them from the agricultural system. everything else I can think of is the equivalent of hopes and prayers.