I'm not going to add anything new, but just try to explain in terms understandable to mere mortals. First, the rule that werden takes a nominative object is an oversimplification at best. Some verbs in both German or English describe the subject as either a adjective, noun or some other phrase. Examples in English include: "Jane is smart." "Jane is becoming a teacher." "Stay there." In German, "sein", "werden" and "bleiben" can be such verbs, though they can have other meanings and uses. When it's a noun in German, the rule is that noun is in the nominative case. The most famous example is JFK's "Ich bin ein Berliner." Note that it's not "einen" or "einem", just "ein", so nominative case. (Technically, correct English uses the same rule, but very few people actually follow it. I once saw a character in a TV show ask "Are these they?" This is actually correct but the sentence was immediately ridiculed by the people he was talking to. The idiomatic form would be "Are these them?") But it doesn't have to be a noun at all, and the case rule would obviously not apply then. Actually, your sentence is not an example of this since "werden" is only being used to form the passive voice. In English with the active voice "Something triggers the process more by the clearing of an existing forest stand." This becomes the passive: "The process is triggered more by the clearing of an existing forest stand." As I said, "werden" can have other meanings and uses.