I screenshot Yellowbridge. If I had to guess, is the etymology related to the fact that clothing can be mixed or piled up?
杂/雜 originally refers to assortment of colors, referring to clothes that are multi-colored. You could think of it as multiple cloths of different colors sewn together to create a multi-colored garment. Its meaning is closely related to 集 as well. From there, the definition evolved to mean 'mixed' and 'miscellaneous'.
集 is doubling up both being the phonetic as well as semantic component for 雜. 集 refers to a gathering/assembly of birds (originally 3 birds 雦). A tree is where birds often gather.