Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means sink so the verb vocab version is to sink.
This is an intransitive verb, so it refers to something sinking on its own. You didn't do anything to make it sink. You know this is when something is just sinking on its own (you didn't sink it), because of the む, which shows it's a cow sinking. The cow stepped into some quicksand and now it's sinking, not because of you.
You look at the cow. Poor cow. She's (しず) sinking.