What was the new culture of power-sharing developed after 1990 ? Or When was the beginning of Coalition Era and Why?

The new culture of power sharing developed after 1990 with the rise of regional parties in many states.
• The culture marked the beginning of coalition era.
• The Era of Coalition Government implies that if no single party gets a cfear cut majority in the Lok Sabha, then the major national parties enter into an alliance with many parties including regional parties to form a government at the centre.