Parkinsonian symptoms can appear in about 50 percent of patients taking conventional antipsychotic drugs because:

Parkinsonism is a bunch of symptoms that mimic Parkinson disease. Parkinson disease itself caused by low level of dopamine. In contrast to parkinsonism, schizophrenia is caused by high level of dopamine, thus the antipsychotic drug is working to decrease dopamine. This often causes the dopamine became too low and result in parkinsonism