There's a few things going on here;
Sidious is keen to foment war and discredit the Senate.
Having the Queen defeat the Trade Federation and drive them off of Naboo is a big step in the wrong direction. By comparison, having her capitulate and, under obvious duress, sign an unfavourable treaty will result in endless indignation from the more democratically-minded Senators and further convince the populace that the Senate is hopelessly corrupted.
The Neimoidian’s flat, reptilian countenance managed a small flicker
of humor. “The Naboo and the Trade Federation will forge a treaty that
will legitimize our occupation of Theed. I have been assured that such
a treaty, once produced, will be quickly ratified by the Senate.”
“A treaty?” the governor exclaimed in astonishment. “In the face of
this completely unlawful action?”
Phantom Menace: Official Novelisation
As mentioned elsewhere, Sidious is keen for his plan to be successful, and not just as a way to gain the Chancellorship.
Maul's boss senses something
Sidious sends Maul to be his man on the ground, supervising things while he can busy elsewhere, presumably working on his maniacal laugh. He also, evidently, senses that the Jedi will have some sort of plan to help the Queen. Maul is going to take care of them.
The faceless speaker nodded. “Good. I will see to it that in the Senate things stay as they are. I am sending Darth Maul to join you. He will deal with the Jedi.”
Phantom Menace: Official Novelisation
This was always part of the plan.
Maul is keen to reveal the presence of the Sith to the Jedi, to demonstrate that they still exist and are superior. Sidious evidently sees little downside to letting Maul off the leash for a bit now that his plan is in motion, especially given what he judges to be the relatively low risk.
Darth Maul exhaled sharply. Satisfaction permeated his voice. “At last we will reveal ourselves to the Jedi. At last we will have our revenge.”
“You have been well trained, my young apprentice,” Darth Sidious soothed. “The Jedi will be no match for you. It is too late for them to stop us now. Everything is going as planned. The Republic will soon be in my control.”
Phantom Menace: Official Novelisation