Workflow loops can happen if you use an app that can be a trigger as well as an action, e.g. with GMail.
For example, if you just use "Order" in the subject line to filter incoming sales orders to your "trigger" Orders folder in Gmail and you created an outgoing email action with the word "Order" in the subject of the outgoing email. All email with the word "Order" in the subject will be moved to the trigger folder, even email from the Sent folder and thus creating an endless loop.
To avoid this, improve your filters in Gmail. For example, create a filter in Gmail on the subject AND on the sender And on the receiver.
Note that if you created a filter in Gmail, not just a new email in the inbox but also a new sent email is filtered.