Number of reasons I can think of or have heard:
1) Could be a nerve problem somewhere in ya neck or something.
2) Could have low-blood pressure.
3) Could have...
My first one was f*cked up. There was no 'silent' mode so the farking thing would wake you up anytime of the night barking (was a dog) and you'd have to feed it.
I tried taking the batteries out of...