What kind of pump is it? It's possible it's just reacting to the pump discharge pressure fluctuation.
Edit to add: I see it says diaphragm pump. I'd guess that this is normal discharge pressure. They work by pushing a volume of water, refilling (pressure drops off) and then pushing out the volume again. In other words, the discharge pressure won't be particularly constant.
See here:
Who knows what the construction inside your pump is, but the principle is likely the same. If the gauge wasn't doing this before, but is now that might indicate something is wrong.