The difference between a 7 and an 8 are not enough to cause that. I'm guessing your forward pressure is off, sometimes you wear down the toe of the boot slightly and this causes the forward pressure to be off, causing a pre-release. I recommend you take them to a shop and have them look at it.
If it's not that, it's cause you're not popping (but it looks to me in the video that it's happening as you set the edge).