Sudden increase in timing rate?

[dchang81]

Have a watch that was pretty accurate. Consistently was around +2 s/day. All of a sudden now it's +17s/day. Doesn't seem magnetized, no drops or bangs. It's very precisely +17 to 18 for the last month. Is there anything specific causes for a rate change that's stayed remarkably stable? The obvious answer is send it for service, which I will but never had any watch do that