The Usual Tactics of the Isaiah Society

Ollivander had once been the victim of a burglary. It was only after confirming Wizet's identity as a "Guardian" that Dumbledore revealed the details;

The burglar had been a Canaan Wizard, and the item he stole was the "Wand Ledger," which recorded information on wizards and their matching wands;

Although the Canaan Wizard was later captured, the Wand Ledger was no longer on him. He had also been hit with a powerful Obliviate, losing all of his memories.

Besides that, Selina Pendragon, the previous Guardian, had written Wizet a letter containing this sentence:

[The moment I sensed that you had obtained your wand, the Isaiah Society likewise noticed the disturbance in Diagon Alley and realized that a new Guardian had been born. They also obtained your name from Mr. Ollivander.]

Combining the information Dumbledore had revealed with the letter Selina Pendragon had left behind, it was easy to draw a conclusion—

The pure-blood family members planning to march and demonstrate were not in league with the dark wizards who had set fire to the camp;

The Isaiah Society must have discovered that someone was preparing to cause trouble. They then used the Imperius Curse to control a group of dark wizards and, while the pure-blood family members were marching, escalated the situation through arson to create chaos.

As for why the Isaiah Society had done this, Selina Pendragon, who had been fighting them all along, was obviously more familiar with the reason.

With that in mind, Wizet organized his newly formed deduction and wrote it in Selina Pendragon's Notebook.

Selina Pendragon replied faster than he had expected. [Not surprising at all. The Isaiah Society is exactly that kind of group. Otherwise, how could they have wandered for a thousand years?]

[They are like Dementors—they love creating chaos and savoring it.