Complex sentence: What was happening was the decomosition was catalyzing itself, ("autocatalysis ") which waswhy the reaction seemed to speed once it started; the mere presence of tin pest led to"more" tin pest.
Split into simpler sentences: What was happening, the decomosition was catalyzing itself, ("autocatalysis "), why the reaction seemed to speed once it started. The mere presence of tin pest led to"more" tin pest.