There seems to be an odd attack rotation between Space Cowboy and Tater Toss. When both are in the same lane, Space Cowboy moves prior to Tater Toss going off. I realize that the info on Space Cowboy says it will move once it hits the plant hero. However, with this same feature against other plants, the plant gets to attack. I don't believe this is a fair mechanic for the game. If PopCap plans on leaving it this way, please let the community know so we can adjust our strategies.
As much as it seems unfair, it's probably best it's kept the same for the sake of consistency. The difference in this case is that the hothead exploding isn't the same as it attacking. Much like Space Cowboy moving, it is an extra effect, which will always happen after the extra effects of the zombie on that lane.
I believe it goes:
Zombie Special Effect
Plant Special Effect
Zombie Bonus Attack
Plant Bonus Attack
(The last two are speculative, but based on the fact that Plant Special Effects appear to take place before a Zombie's Frenzy triggers.)