When we eat, walls of our digesting system move, pushing the food down, towards our stomach. However, there are some factors which can make the food come back, for example:
Bacterial infection → our body wants to get rid of the bacteria ASAP
Travel sickness → it can cause minor nausea connected to movement
'Weak Stomach' → for some people, mixing some types of food causes vomiting
When a person vomits, contents of their stomach are pushed back upwards by reversed movement of the muscles of the pipe. At the same time we can observe extremely abundant production of saliva which is supposed to protect our teeth from the digestive acids.