Cheaper flights - It will increase the tourists average rate suddenly and more people will travel because of cheap flights.
Globalisation - It would connect people from various places and will be seen as an influencing point and more tourist attract to that site.
Other causes are rapid urbanization, lack of hospitality services.
Effects:
Pollution - Overcrowding will increase pollution factors like water pollution from litter or plastic waste, Air and Noise pollution would also see a drastic change.
Land Degradation - Overcrowding especially because of Tourism will lead to land degradation for example, deforestation by local tourist government agencies due to the rising level in tourist numbers.
Other effects would be security, environment and social issues.