I have witnessed this more than once, particularly with the Ghost and K-2 EMF Meters. During an investigation, three EMF meters are placed next to each other on a table. One meter will keep going off while the other two remained silent. The investigator in order to prove that it is something paranormal will change the position of the EMF meters. Again the same meter will continue to give an indication. The investigator will conclude that it has to be something paranormal causing anomaly because only one meter is indicating the presence of an EMF field.
What I think is really happening is that the meter indicating the presence of an EMF field has a weak battery. During my career as an Aircraft Mechanic, I would use a digital multi-meter every day. I sometimes noticed when taking a reading, the meter would fluctuate slightly and not settle on a particular value. Through experience, I learned this was an indication that the battery in my multi-meter needed to be changed. As soon as the 9-volt battery was replaced, the meter would act normally.
I think a weak battery is a cause when an EMF meter starts giving a constant or fluctuating indication. If you have two EMF meters and one keeps going off, change the battery. If the indication persists, then maybe you have something paranormal. If the constant indication quits, then verify the battery voltage to see if you have a weak battery.