It is not always related to Karma - though it can be. It usually happens when a person is in a weak or vulnerable state. Negative spiritual entities are much like traditional predators we see in the animal kingdom. They go for the easiest targets and so when someone has gone through a difficult period in their life and is in a low energy state - they will see this as an opening.

These weak and vulnerable states …. can of course be brought on by poor karmic choices (drug use, sexual immorality, theft, incarceration etc etc) …. but that is not always the case. It could also occur when people are simply under a great deal of stress from work, have experienced great trauma or sadness in their life like the end of a relationship or death of a loved one or they are simply in a weakened physical state for some other reason.

When we are in a negative state of mind or under great stress - our energy body and state of being is more “in tune” with theirs. Thus they are able to more easily “hook into” our energy body to transmit their own negative energy into ours to then influence the person thinking patterns and emotional state. The basic premise is to continue bringing the person down to lower and lower states so they can take more and more control. Confuse their thinking - bring on negative thinking and behavioral patterns so they can then maintain this connection and control. - David Agry https://www.quora.com/Is-getting-possessed-by-a-demon-because-of-your-karma

On curses ...

"When we use force, violence, deception and exploitation, we automatically get cursed. Remember that exploiters always get exploited; fury burns the one who is furious. People having occult powers use them to help or harm others. Taking recourse to such people and their powers may give temporary respite or support, however, it sets into motion a vicious circle of karmic debts that further aggravate the misfortunes of those who seek such help and also those who misuse their powers." - http://www.newindianexpress.com