because zac from all the tanks so far has the least Things that fit his kit compared to the playstyle that was meant for him. zac is THE example when you say "tanks with too much damage" and for someone who has a very great initiation potential he just feels so lack luster once you used your E and R and all his Q and W really do is damage a bit, his Q gives a mediocre slow, but his W is nothing but pure melee ranged damage and for a tank that is supposted to dive in, fuck shit up, and stick to fucking shit up, it does not fit. AT ALL. zac out of all the tanks you mentioned has the biggest Problem with it, so i assume that is why he was put high on priority.
same with sejuani. you literally Play her for one Thing alone, which is her R. thats the only very reliable Thing she has as a tank, her Q dash is very small and hard as fuck to land, her W is just like zac's W, pure damage, her E is somewhat arguably nice but situational since it has a comparably high CD and the slow can only be applied upon activation. Zac and seju are the kinda champions you really Play for only one single Skill, zacs E and seju's R. Malphite you can use to shut down comps with his E and R and Q, he is useful with all skills, same with rammus, his Q can be a massive threat, his Ult is very unexpected despite being a damage only Skill aswell, but it is AoE centered and since you will be taunting people hard and possibly apply at least one more Kind of CC you can Keep his ult up for heavy Extended damage. Volibear is very good to disrupt a chase and hunt people down, his Q makes very good use of his kit as a whole, it can initiate, you can perfectly control who you will pick out, and thought voli is probably also like zac and sej a champ you Play for only one Skill (his passive for the stupendous tankyness) he Plays fairly well and does his job as an Initiator better than the 2 mentioned earlier, and since his Q and E have both low cooldowns compared, and his E is AoE aswell, he serves a much stronger use than them.
Yes, voli is neutral territory because he also falls into this category tanks with too much damage, so i suspect riot will put him into their high priority list soon, possibly next. but for the other tanks you mentioned, here is my personal opinion on how riot could see this case
Edit: malph and rammus are also listed for the rework candidation, thought zac was only mentioned as the first one for a topics sake >_>