I notice that you argue that it couldn't have been about her wanting to put the tea ball in because whether you or she did it would have had no impact on the objective outcome of the tea. That makes rational sense and is how a normal person under normal circumstances would see it.

Then again, stuff like the IKEA effect is real: people feel differently about a piece of furniture if they assembled it themselves, perhaps there's a bit of pride and feeling of self-worth in having done that work. That feeling is independent of the usefulness of the furniture.

In a similar vein, perhaps your wife feels the need to do certain things herself to feel in control of part of her life. Everybody wants the feeling of agency to some extent, some more, some less. Again, it's almost certainly not about the tea, but if she feels that her life is "taken over" by the relationship (and so, explicitly or implicitly in her mind, by you) and that she doesn't have enough time and space that's her own, then something small like the tea could trigger a reflex in her along the lines of "why does it always have to be like this?"

The tea is an almost ridiculously tiny symbol for asserting herself, but it's possible that she feels she has already lost the bigger ways of asserting herself and reaffirming herself in the relationship, and so she reacts badly to that small thing being taken away.

If that really is the situation, it is already a very bad state for the relationship to be in. Of course, I may be way off base.