The Enable Queue Processing button remains enabled while any completed transfers remain visible in the queue.
This means that, by default, if your queue completes a file operation, and you set up another transfer of some sort that will run through the queue within 15 seconds of that operation's completion, the new transfer will initiate immediately, because the Enable Queue Processing button is still enabled.
For me at least, this is unintuitive: I do not expect processing to remain enabled after all transfers have completed, or to depend upon the visibility of *completed* transfers in the queue.
The workaround is to go into Options and change "Transfer - Background - Display completed transfers in queue for" from "15 seconds" to "Never." This leaves you without the option of having those completed commands stay visible briefly after their completion, though.