I have to also assume this has already been considered in the game theory. So something else unexpected, unpredictable perhaps.

Edit: The more I think about it, doing nothing would be most optimal. Doing nothing is so much more cost effective when you know it's a inevitable decline.

Why would Apple care about Spotify's stock price?

Well it's a really dirty tactic, but it can be hard to retain top talent (who are largely rewarded with options/shares) if a company's stock price crashes.

Otoh your chances of hiring that talent are lower if you're the aggressor responsible for the stock tanking

heh true, they might not care at all - Apple makes more $$ in a quarter than several Spotify market caps. But, here's a few theories: if Spotify keeps calling itself the "leader" in streaming and Apple decides it wants this to stop, it would be an easy/cheap way to impact Spotify's brand negatively, and even potentially bring them down entirely. Alternatively, by driving the price down, it could also force Spotify to explore selling itself at a low price (see: Pandora), and if Apple wanted some fast/easy subscribers, it could acquire them cheaply.

One way to leverage a high stock price is through all/part stock M&A transactions.