Tip-in is a good word to call it.I call that moment "tip-in". Every board has a different tip-in feel, some very abrupt. So I'd say it is the board, but also some of what you are feeling is working through that tip-in on an unfamiliar board. Does it happen more on toe or heel, or both? I ask because one time by mistake I set the front up favoring toe and the board wanted to brake and turn uphill.
Happens to me only on toe side. I've chalked it up to me not really getting fully on edge when on a flatter traverse, or possibly being a little heavy on the nose when the trail didn't support it.
Took the board our cat skiing the other day. Man what a dream. Just a shame the virus has brought an end to our season here. The dart is an amazing surfer, and I feel like I just got to understand that over the past 5 or so days on it.