All pony loves Apples, and Chrysalis is not exception, so why not eat one)
Only one thing Chrysalis didn't think about - Changelings copy only outer form.
So... she should have been a bit less enthusiastic about eating apples.
Unfortunately for her Shining saw all and possibly started to suspect something so she had to use.... magic^^
Nothing and no one dares to prevent her plan!

And one more random idea from my head, Just for fun)

Also, one more random thought was about why Chrysalis just didn't eat real Cadence?
Too much calories -__-