Depending on how you have your settings, the chart orientation will be shown on screen as 'North Up' or 'Course up'. It sounds like you may have it set to Course up, which will make it so whatever direction you are pointed is at the top of your screen. I find it can be useful for navigating at a bit higher speeds, but if you are at rest, drifting, or trolling around, your course will change constantly and your screen will rotate like crazy. Switch to North up in these circumstances and your chart will remain fixed with North always at the top of your screen.
Adam