Although x was pronounced like English sh long ago and in fact some dialects now pronounce Spanish ch like English sh, I highly doubt such a link in pronunciation had anything to do with it. That said, for some languages that Spanish speakers are likely to know or be familiar with (like Catalan, Galician, Asturian, Portuguese, and perhaps some American languages), x has a sh pronunciation, and they may have picked it up via those languages — when ñ isn't available, most people switch to ny or nh, for instance.
In reality, x (along with k and w) is not a super common letter in Spanish. And in texting, you're limited to the number of letters that you can use, so it behooves you to reduce the total number of letters where you can. Using x for ch (or w for gu/gü or k for que) is an easy way to do that for arbitrary words without significantly affecting readability.
It's surely laziness for some people but frankly I find it harder to type with such abbreviations, and only use them when something is very informal and I need to save space. That was often true when they charged 0.15 € per text message, but nowadays is rarely useful.