SX50: How Could I Have Improved This Shot?

I think it's the servo AF that matter besides the shutter speed.  The camera is keep on adjusting the focus while you're framing and the focus might went out from the squirrel to the grasses.  Try use the single AF and half press shutter to lock on the target for framing.  Perhaps the servo AF is best use for targeting object with clear background only?

P.S. My experience the focus box will only move by itself only when you're using Auto mode (where there is multi-poiints focusing)

cn