At 6 Months old, my baby is still vomiting milk after every feed. He is a fully breastfed baby. What is happening?

Experienced the same thing with my baby. I can even remember we rushed her to the hospital because she went purple after she spit a lot even through her nose. Thank God nothing happened to her! After this terrible experience, I rechecked myself what could have been the wrong thing I did that caused it - realise that 1 major thing is not letting her burp after feeding, not EVERYTIME. Sometimes I put her down immediately after the sesh. Which is a big no no. Or maybe did burp her but did not take enough time. Now she's 15mons, no more spitting happening but I still let her burp by gently patting at her back. I think mum above made good explanation on what's going on already. So this jts from someone who experienced it too and now less worried after knowing the how's and why's. :)