Theory: When Tortimer retired, Isabelle wanted to take over, but was refused. When she realized nobody in the village knew what the next mayor would look like, she came up with a plan. Isabelle, being a mayor's secretary, had access to the information about traveling by train. She found out who the next mayor was, as well as the child arriving to live in the village. She sent a message to the new mayor that there was a change of plans and someone else would take over, causing them to cancel their trip. (This is why they never seemed to arrive in town.) Isabelle didn't want to risk trying to convince the villagers that she was supposed to be mayor, so instead she decided to put those responsibilities on the other person, the child.
She knew that since the child would much rather have fun living as a regular villager, that would leave all the mayoral work to her. Plus, being an adult, she had an authority over the child despite them being her superior. So, when the child arrived, Isabelle convinced everyone it was the new mayor, and would not let the child refuse. Now that there was no actual qualified mayor in place, all Isabelle had to do was grant the child a few special abilities (ordinances + pwps) and she would be the unofficial mayor of the village at last.
Fortunately, Isabelle isn't evil or anything, so her plan to take over the village doesn't actually affect anyone else too much. Maybe Tortimer ended up being an awful leader, and when he was close to retiring Isabelle saw her chance to change the town for the better, and sabotaged the new mayor's trip to the village because she couldn't trust anyone that Tortimer did.
this probably isn't even close to plausible but...let me have fun