My understanding is that churches were built next to yew trees, not yew trees planted next to churches.

Pre-Christian religions had many associations with yew trees (they live for a long time, give off mildly hallucinogenic gasses on hot days, discourage animals), and so built their holy sites around them. When Christianity came to Britain, churches were deliberately built on pagan holy sites to overrun the old religions, in the same way that early Christianity took over roman holy days (Saturnalia -> Christmas, Lemuria -> All Saint's Day). This led to churches being built next to sites with copious yew trees.