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I have noticed some people from Andalusia pronouncing the combination "st" in reverse in a word, as "ch" (for example "donde está" will sound like "donde echá"). Is it a particularity of the local accent of some kind of slang? In what areas, if any, can this pronunciation be heard?

This affrication of /st/ is indeed particular to Western Andalusian:

An affricated dentoalveolar stop [t^s] (listo [ˈlit^so]) has been described as a variant of /st/-clusters. This sound is perceptually and acoustically similar to [t^h], another /st/-allophone in Seville Spanish.

• 
  Affrication of /st/-clusters in Western Andalusian Spanish: variation and change from a sociophonetic point of view, Hanna Ruch (2010)

The study reports that it more often occurs in younger, educated speakers, and in informal speech:

The frequency of occurrence of [t^s] was negatively correlated with
age and speech formality level, and positively correlated with the level of education. The social distribution of [t^s] suggests a sound change in progress in Seville Spanish.

The study posits the novel sound change: [ht] ⟶ [t^h] ⟶ [t^s] (noting that this affricate has not been described in the traditional dialectological studies).

Also note the distinction between Eastern and Western realization of /s/ aspiration when it occurs before consonants (pre- and post- respectively):

In Eastern Andalusian Spanish, /sp, st, sk/ clusters are usually realized with preaspiration [hp, ht, hk], whereas in Western Andalusian Spanish, postaspiration [p^h, t^h, k^h] is much more frequent, at
least among younger speakers...