As reported by ARS Technica, this is primarily because Apple has been spending a lot of money on stock repurchases. Apple had $163 billion in cash reserves in 2017 and has spent $122 billion on stock buybacks since the start of 2018. The company also took advantage of the Trump tax cuts to repatriate cash it held overseas.
Virtually every Apple business grew in every region this quarter—except the iPhone Since then, Apple has worked to reduce its liquidity in response to criticisms from investors that the company was hoarding cash. While there is some room for nuance, investors generally want to discourage companies from hoarding cash. Instead, they prefer that the money either be returned to investors in some way or invested in something that may produce a return later.
Apple spent $122 billion on stock buybacks since the beginning of 2018. It also took advantage of a one-time tax incentive to bring overseas cash back into the United States at a reduced tax rate a little over a year ago.