i'd say this would be an isolated issue for sure.
when it "rings" is it a random ring tone or the one you have setup for calls? do you have different ringtones for different contacts or just one general ringtone?
was it connected to any bluetooth gadgets? any headphones plugged in? i know you said this happens when you plug it in, but is this the only time that this happens? do you plug it into the cars cig lighter or does this happen or when you are home/etc?
try a different cable? does this happen when you plug it into a computer usb port? wall? phones cig lighter?
are you rooted? and what carrier?
you have the "nexus s" as you current phone. are you speaking about the nexus or the htc one?
can you replicate this issue at will?
like i always say, it can be anything. all it takes is for one tangled app or glitch betwixt two apps or so and anything on these supercomputers can happen.
if you installed any apps prior to this happening, remove them. if you deleted anything then find out if it left any remnants of its install.
I asked a lot of questions, but i'm just trying to get the juices flowing and help you to track it down.