Why were the linguistic group unhappy …

The linguistic groups were unhappy withe the congress since the congress conceded with the demands  that states in India should be organized along linguistic lines.  After independence, the national leaders opposed and resisted the demand for linguistic states. This made the linguistics group unhappy. The linguistics launched a movement for a separate state for the languages they speak.
Many people in India derive their primary identity by the language they speak which includes Tamil, Gujarati, Bengali, Andhra and many others. Before independence the congress party conceded to the demand of separate states in principle.
But after independence the national leaders opposed and resisted the demand for linguistic states, since these methods would let loose forces of national disintegration and disrupt economic reconstruction.