Why Shakib Al Hasan has been banned from bowling in int'l cricket?

On Sunday, December 15, Bangladesh all-rounder Shakib Al Hasan was suspended from bowling across all top-level cricketing tournaments. As per the assessment conducted at Loughborough University, it was found that Shakib's "elbow extension in his bowling action exceeded the 15 degree threshold as defined in the Regulations", leading to his suspension followed by ECB's decision.