What's the meaning of this sentences with 丛, 纷纷扬扬 and 飘来飘去?
丛 just means "thicket". What did your dictionary say?

纷纷扬扬 is a fixed expression meaning "fluttering (of leaves)" exactly as the rest of the sentence describes. A good dictionary should include fixed expressions like this. The first part 纷纷 does mean "in succession", but it's not usually useful to try to break down fixed expressions to understand the literal meaning.

在空中 is in the air, yes.

X来X去 means "X back and forth". E.g. 走来走去

飘 means float or flutter. Have you tried looking things up on https://www.mdbg.net/chinese/dictionary ?