I have no idea what causes it but I seem to have a similar situation. I am usually fine in the morning, until about 4 or 5 pm. Then they start up. Last year when I was having major problems it did sometimes seem related to what I was doing. If they started up really bad I could sit down and after a few minutes they would get better. Stand up and they would start again. Sometimes lying down would stop them. It was very weird and I asked my cardiologist and he didn't have an answer.
PACs are very often triggered by adrenaline and similar substances. PVCs are too, but the ventricular cells are less sensitive to adrenaline than the atrial ones.
By shifting position from sitting to standing, you will at least double your adrenaline levels. During the day, you may increase your stress levels, stress at work, food will increase the adrenaline levels, etc. I have the same problem. They appear with stress, often in the afternoon and evening. If I exercise in the evening, I can get them. I rarely do if I exercise in the morning.
Losing weight might help. Better exercise condition might definitely help.