Heat and especially humidity affect the wood of an instrument, causing it to shrink or expand, so the strings go out of tune. This is particularly noticeable on a mandolin, but it happens on [acoustic] guitars too.

Why does my guitar string shake so much?

If you hit the strings too hard when strumming, it can cause the strings to vibrate up and down too much (as opposed to side to side), increasing the chance of buzzing.

Is a little fret buzz normal?

Because of different style preferences, some players are okay with a little fret buzz as long as their action is as low as possible. However, others may find even a little fret buzz distracting and uncomfortable. ... If the pitch doesn't change when playing adjacent frets. If you can hear the buzz through your amp.