The most major problem is of the Hunters. Though Hunting is banned, there still remain some.

Also, deforestation poses a great threat to their existence. Jungles are their natural home. So, if their natural home wipes out, where will they live?

Pollution also affects the tigers. They face nearly every kind of pollution (except Space Pollution)  like Soil, Air, Water And Noise pollution. 

Biological Magnification is another major problem. Due to this phenomenon, amount of non-biodegradable chemicals is increasing in the bodies of not only tigers, but all the animals, including Humans. 

Tigers also face water pollution to a great extent. Rivers nearby human civilizations go into forests. So, pollution done by us affects them.

Tigers live in a world where "only the fittest survives". They often are in close combat with other wild animals, especially when young. This can be considered a danger, but this one's needed.

Last but not the least, Global Warming and Rising Temperature affect the normal living of tigers.