If memory serves me well, in one of the letters he sent a number of years back (10-ish?), he mentioned that they had someone in their staff managing a portfolio of stock picks and doing a great job at it, and gave a heads up to readers that they might find tech stocks come in and out of their portfolio every so often as a result. Presumably he liked the returns he was seeing and jumped in too.

Also, Apple displays a number of characteristics that he reportedly likes to invest in: the business that Apple is in is straightforward to understand, the market is big and they've a strong or growing market position, they're profitable and have a big cash position, they've a strong brand and difficult to reproduce expertise (i.e. a moat).