It seems like the British spent many years cultivating a religious movement among the Indians in Ohio. Tecumseh's brother was a Prophet with a religious teaching that was suspiciously in line with British Empire goals along the United States frontier, which at that time was Ohio. Tecumseh's movement looks a whole lot like the proxy armies fighting in the middle east today--spurred on by religious teachings and financed by empires who are far removed.
The British had repeatedly betrayed the Indian tribes leading up to the War of 1812, and for several previous decades, also the Empire had very limited ability to deliver on any promises it would make to the Indians. It's really remarkable that they were able to persuade Tecumseh and his people to undertake a war against the Americans.