Why was the game show host so keen to have Jamal lose in Slumdog Millionaire? Did he have a financial stake in the show? This was never explicitly stated, though at some point he does say "This is my show." Though this statement can be interpreted in any number of ways.
I guess there was a dialogue where the host said and I don't remember the exact dialogue, but it was something like this
"No chai wallah can come from the slums and become a millionaire. Only person has done it so far and that's me."
I am pretty sure there was such a line which establishes the fact that the host himself had a rags to riches story and he started to envy Jamaal when he saw him winning.
I believe that "this is my show" is meant to tell us that the host wants to be the show's only star. Due to his success and back story, Jamal was becoming very popular. The host knew that should this chaiwalah win the ultimate prize, his popularity would explode and eclipse his own. Jamal would become the show's biggest star overnight. The host's ego couldn't allowed that to happen.
The interesting thing about the film (and the book it is based on) is that Jamal knows answers to questions (and many of them in a row) - which he should not have known otherwise.
However, due to co-incidence, there have been situations in his life (fortunate and unfortunate) which cause him to know the answers to all the question (and this is what the film explains via the flashbacks).
In fact, the synopsis of the book on Amazon reads:
Vikas Swarup's spectacular debut novel opens in a jail cell in Mumbai, India, where Ram Mohammad Thomas (Jamal in the film) is being held after correctly answering all twelve questions on India's biggest quiz show, Who Will Win a Billion? It is hard to believe that a poor orphan who has never read a newspaper or gone to school could win such a contest. But through a series of exhilarating tales Ram explains to his lawyer how episodes in his life gave him the answer to each question.
Hence, I feel that the game show host strongly believed Jamal to be cheating - as in his mind, there is no way he could know all the correct answers (including the one that the host himself writes incorrectly on the mirror in the bathroom).
Hence, he wants Jamal to lose and also the reason as to why he gets him arrested.