What causes mucus from the pushing side and back of neck to come out of my throat?

See the below diagram of our sinus cavities.

They are all connected. If you drop oil in ears, it comes out from your mouth or nose. If you drop oil in nose, it come through ears or mouth. Google it; you have seen people squirt milk from eyes etc. (Guinness even has a record for it) . Its not magic. Its human anatomy. Yes, hard to believe. So what you are doing is you are directly ensuring that you remove the blockage or mucus.

Try avoiding, its not the right thing. Instead, gently blow both your nose. Regularly sucking mucus out from your mouth or blowing a single nostril hard can bend your sinus cavity bones. These bones are flexible, so if the bones in nostrils or sinus gets bent, then they will cause blockage leading further complications like sinus, deviated nasal septum. You will easily catch cold. So, be gentle. sometimes, i know you get a oily feel on the back of your throat, then yes, cough and suck out the mucus from your nose. Otherwise, don’t forcibly do it. As i said try to blow GENTLY and always blow both your nostrils.