Database logs growing during maintenance plan
What I am trying to understand is why this is occurring, and how to prevent it. 


Maintenance Plans do not have much logic that you can implement when dealing with index fragmentation and once you configure it, they run regardless if you really need a reorg or rebuild on your indexes. 

You are better off implementing Ola's backup solution as well as Index maintenance  solution .

Also, refer to Why Does the Transaction Log Keep Growing or Run Out of Space? - especially answers from @MikeWalsh and @AaronBertrand