Could she have smelled where a squirrel dug out an acorn or something like that?
Mine used to love to dig, dig, and dig! I'm not kidding, either, when I say she nearly destroyed her fenced-in yard with the little holes she left all over it the first summer I had her! But we lived through it and now, just occasionally, she'll dig a bit where it's bare around a plant.
Get her a sandbox! Yorkies do love to dig!