Separation anxiety is a common cause, was she left alone? The theory is that as she's got older and more confident in her mind she's taken on the role of looking after you. When you go out she can't find you and goes out of her mind with worry that she's lost you while she's supposed to be looking after you.
My first dog started doing this and when I looked a bit closer I noticed he started chewing the last place he saw me (the back door). I got home once to find he'd chewed his way out and was waiting for me in the garden!
I solved the problem by getting another younger dog for him to look after (worked straight away) but I was planning to get another any how. You should be able to solve it by relieving her of her 'responsibility' - to do this you need to convince her that you are the 'alpha' dog.
There's a really good book about this and other causes of behavioural problems in dogs called "The Dog Whisperer" - it changed the way I behaved around dogs completely.