I think it honestly depends, really. When my mind goes off on a tangent, it often reminds me of something negative for no reason whatsoever & then being upset kicks in after it's had some time to sink in... But there are other times where I get stressed out due to an event & THAT'S when my negative thoughts start happening. The way I figure it out for myself is by asking "do my thoughts have to do with something that upset me, or are my depressed thoughts all over the place & I'm just grabbing at smoke?" The former is triggered BY the thought, as it's a linear path. The latter is triggered by emotion: You know HOW you're feeling & you're just overcome by a slew of thoughts having to do with that emotion.