When you exercise your heart is getting signals from two sources to beat fast. One is from nerve cells from the brain. The other is from hormones, the endocrine system, in particular adrenaline. You have some cells in your heart predisposed to ectopic beats (called enhanced automaticity, or EA). This is not uncommon. Nerve signals and/or adrenaline can make EA more likely, hence you will get more when you exercise.
This is just one logical explanation. There could be others. Don't worry about it. I've been getting them since I was in my late teens and I exercise like a crazy man. 48 now and still healthy heart.
When you think of the things you imagine it might be, follow that all the way through to the management of the issue. In the case of a-fib, PSVT, and NSVT, there really isn't any way to manage them, or prevent them from getting worse, if that's what it is.
That's what I learned when I was in the "a little bit of knowledge is a dangerous thing" stage. I went through imagining it was all sorts of bad stuff. In the end, managing what I thought it was isn't possible with medication or surgery. So I moved on and didn't die yet! I just soldier on.