Phenomenon is known as Secondary mutation. This is a phenomenon which among mutants that seems inexplicable, with many previously established mutant characters suddenly developing new or vastly expanded powers
Some of the known mutants having secondary mutation are:
Gambit (Remy Lebeau): Gambit is able to psionically suggest things to
people using his hypnotic charm. It allows him to exert a subtle
influence over sentient beings, leading them to believe what he says
and agree with his suggestions.
Polaris (Lorna Dane): after having her mutant powers leeched by her
supposed-sister, Zaladane, Lorna developed the first recorded case.
Her body mass had increased, along with her strength and resistance to
Emma Frost: Emma's secondary mutation activated in the midst of the
bombing of Genosha by Cassandra Nova's wild Mega-Sentinels. She
transformed into an indestructible organic diamond form that saved her
life, but negates her telepathy so she cannot use both powers at once
There are many more. Follow this to find more details