Source: soruce: Mail Online
The illegal pet trade of the cubs of the Sun bears in the Southeast Asia is another one of the causes of the alarming decline in the Sun bear population. There is a demand for these small cubs as they are appealing to people, and since the law does not allow the Sun bear to be kept as pets, people get the cubs illegally. To obtain the cubs, the mothers are usually killed so that the poachers can get the cubs more easily without getting harmed by the protective mothers of the cubs. However, the cubs will grow into an adult Sun bear and become more aggressive and dangerous to keep as they grow. This is when these grown sun bears will be surrendered over to the authorities as the owners will not be able to handle the bears’ aggressiveness.
These Sun bears will then be released into the wild, but because their mothers were killed early into their life, they do not get the chance to learn the necessary survival skills to survive in the wild (which is usually learnt from the mothers). This will lead to them being unable to adapt to the new environment they were released into and would not be able to survive long term in the wild.
Source: The Telegraph
Hence, the killing of the mothers and the keeping of the cubs are causing and contributing to the declination of the sun bear population.