[Venger]

Hmm you sure it was a 'mouse' spider? (they usually have a 'furry' abdomen hence - looking like a mouse, rather than the shiny, smooth looking brown spider)
Sadly 'mouse spider' covers quite a large group of spiders Gnaphosidae, Clubionidae have all been called mouse spiders, mostly they are nocturnal hunters, though there are a few that hunt during daylight.
Generally wandering spiders will take what they can get, so this might have just been an oppotunist attack.