I forgot about those particular examples. I suppose it would be more accurate to say that all doomed timelines are separated from the alpha timeline by someone with knowledge FROM the doomed timeline, whether they be Dave or another Time player, Terezi or maybe Rose using Seer powers, John with his new powers and apparently even the Denizens themselves.
Those are the only examples of doomed timelines we've ever actually seen.........
Besides that, there are WAY too many doomed trolls floating around for just that to be the case. Maybe it's due to the recklessness of Aradia and the maliciousness of Damara, but maybe there are more ways to make a doomed timeline.
Pretty sure all the doomed trolls from the Alternian troll timeline was because the Aradiabots, acting on instructions from the cross-timeline bulletin board, intentionally sabotaged a thousand timelines so they'd have the numbers to counteract the Black King's psychic powers in ONE of them. Kingmaking, as it were. And the main reason that the Beta kid had relatively few doomed timelines was because Dave was REALLY careful with his time loops. We know literally nothing about the Beforan timeline, but keep in mind that, after the first dead Dave, Dave was ridiculously careful about keeping his time loops stable and following the alpha timeline. He might be the exception, rather then the rule.
Perhaps simple chance is sufficient? We know a bunch of doomed Aradias were the result of Karkat's husktop not exploding, and Aradia wasn't responsible for that.
Karkat's husktop not exploding WAS choice, since, in that case, he didn't choose to run Sollux's virus. And that whole thing was weird, since if it HAD exploded and a doomed Aradiabot hadn't intervened, Doc Scratch wouldn't have been made since Gamzee wrote part of his genetic code during that doomed timeline.