There's enough space between them all and they're all in the northern half.Are you planting too many of them in a row? Are they too close to rocks or buildings? I assume not since you've said you're following the general rules for planting trees but I'm asking just in case. Also, remember that you can't plant cedars (I think it is) in the southern half of your town, so if any of those saplings are cedar saplings and that's where you're planting them, that could be it.
It's always a different sapling(s) dying so that's why I'm wondering if I'm just planting too many trees at the same time or something