Question: what were the main reasons the bantu speaking tribes left central africa
[{'answer': 'migrations', 'provenance': [{'wikipedia_id': '34055635', 'title': 'Bantu peoples', 'start_paragraph_id': 21, 'start_character': 118, 'end_paragraph_id': 21, 'end_character': 128, 'bleu_score': 1.0, 'section': 'Section::::History.:Origins and expansion.\n'}]}]