Rat on a Air India plane - Air India London flight, called back to Mumbai

On 30th December 2015, Air India Flight No AI 131 carrying around 240 passengers was called back, which was flying from Mumbai to London when it was near Trehan because a rat was inside the flight.

Air India Team is investigating the incident.

It is not really good news for Air India (probably for government of India)

The question is how it happened?