It’s corporate governance. Every share represents a number of votes. The votes control who sits on the board and therefore who runs the company.

Because Twitter is publicly traded, there are enough outstanding shares for an outsider to buy enough and get enough votes to change the direction of the company. This is called a hostile takeover.

A poison pill is a defense mechanism to prevent an outsider from completing a hostile takeover by diluting their ownership. There are a few valid reasons for fighting a hostile takeover and the most common are the asking price is too low or you don’t think their plans are in the best interests of the company.