Not sure if you think you hiatus hernia is related to the cycling issue? You mention it but don't offer how you think they are related.
An amateur theory on PVCs and tach is that certain types could be caused by the sympathetic nervous system. This has never been shown in a study, it's an Internet hypothesis by none-medical folks. That said.... If you buy into this theory I can see there might be a relationship between being bent over on a bike, increased abdominal pressure or thoracic pressure and a bulging hiatus hernia irritating the vagus nerve. This hasn't happened to me on my bike but I have noticed that certain movements that cause these pressure definitely do trigger PVCs.
Another thing to consider is that your heart rate is probably higher when you bike. The higher rate might be what's causing it.
I have a hiatal hernia, and believe me, if I bend over, turn a certain way, my PAC's definitely act up, especially if I do this after eating or drinking water. I am quite sure that they can be related, it just makes sense. Maybe no doctor has "Officially" said it, but, when you read all the posts that those of us have written that suffer with this problem, it is more than likely a very real cause.