The modified date on the parent folder will update every time a document in it is renamed (because under the hood, renaming documents is considered a modification of the parent folder by "moving" a document from its current full path to a new full path). 

For starting rule purposes in Workflow, renaming an entry is classified as an "entry changed" event. That was a design choice because we thought classifying it as a move would be too confusing for the user and would require extra conditions to determine when it's a "proper" move versus a name change.