TL;DR : this is not a standard pronunciation. Even @Gilles can’t picture anyone pronouncing [tʃ] or [ts]¹.

In Français Standard, it should be [sɛtyn], [tʃ] is a common variant in Cajun French and in varieties of rural France, especially in informal speech. Québécois and Acadien use [ts], which is quite close and may be related.

As for why, I am not really sure, but I suspect it is a case of mouillure or palatalisation: it happens in many mothereses in French, and in some cases in informal speech that an occlusive consonant before a front vowel is palatalised as in [k]→[kʲ]

Qui c’est ce bébé ? [kisesəbebe]

Can be pronounced [kʲisesəbebe] in motherese. I have been told by a Basque teacher that this palatalization can be mandatory in Basque when addressing relatives or close friends.

Then, as time goes by, this pronunciation can evolve to [kʃ] or [tʃ]. The same thing happened to the initial [k] Latin cabalus

[k] > [kʲ] > [tʲ] > [tʃ] > [ʃ]

which is how c​abalus became ch​eval.

Since Cajun, and in a lesser measure Québécois inherited more from informal French, it could be how [cɛtyn] became [cɛtʃyn].