There's also the simple fact that there's almost nowhere for the SNP to go but down. Virtually any result would be a loss for them because there are so few seats that they could have but currently don't. All the drama with the end of Sturgeon's tenure and Yousaf's brief leadership will not help them either (plus, of course, just the loss of Sturgeon as a very effective politician).
They do have a ridiculous proportion of Scotland's seats right now, though. They won those seats completely fair and square, but FPTP makes them unbelievably overrepresented compared to their actual vote share. It's a shame that they're the biggest party that wants to actually do something about that