Hard to say...our cats sometimes do this, DH thinks they've just kicked it out of the littebox ITMS? Hopefully it's something minor like that, though annoying to keep cleaning up.

If you do think she's anxious, we use a great product that you plug into the wall called Feliway. Our vet recommended it, only cats can smell it and it keeps them calm, we've been using it since our 2nd son was born and our cats are sooo chillaxed now!

ETA - another thought...have you got the litterbox with high sides? Maybe as she's getting older it's harder to climb in and out of?
can you get that smelling stuff at a pet shop/big w type place?? i need that as my cat IS anxious!

Id say its just her age, if there are no other signs just the litter try. our cat creeps around the house like there is something about to jump out and get her from behind every object! need that calming stuff!!!