That means we can rule out the possibility of Thullatissa simply being a stickler for the rules (namely, that when there are more monks present, there is an oder in which they speak, depending on their seniority (by who's been ordained longer)).
As for Thullatissa's reaction (in that she disrobed), I can think of the following explanations:
1. It seems that she believed that only some monks, at that those that she prefers, are capable of adequately teaching the Dhamma, and that the others are not even worth listening to. While the other nuns were instructed, exhorted, inspired, and gladdened with a Dhamma talk (by Mahakassapa), she didn't have that benefit, as a result of which she disrobed.
2. Perhaps she had unresolved issues about being a nun and thus having to submit to monks (any monk); and thought something like, "If I already have to submit to monks, then at least it should be to those of my choosing. If I'm not allowed that, it's best that I leave." (An issue men don't seem to pay much attention to, but it's something women have to resolve for themselves, somehow.)
3. Perhaps she had undue feelings for Ananda, which she indulged in internally, while not acting on them externally. She isn't the only nun with undue feelings for Ananda (e.g. The Bhikkhuni Sutta, AN 4.159 -- or maybe that is Thullatissa?). Nevertheless, such undue feelings, when indulged in, are a hindrance and can create a lot of problems.
4. A combination of the above.
1. Don't be a drama queen.
2. Allow for the possibility that more people could be able to teach the Dhamma adequately, and not just the one you currently prefer.
3. If you don't take every opportunity to be instructed, exhorted, inspired, and gladdened with a Dhamma talk, this could end badly for you.
4. Clear up and resolve whatever concerns you have about your position in the formal system of Buddhism. Letting those concerns fester unresolved will create trouble for yourself, and possibly others.
5. Acknowledge whatever undue romantic/sexual feelings there may be, esp. if they are for a monastic, and make every effort not to indulge in those feelings, for such indulgence can cost you dearly.