Since the last version of MS Dynamics, a new mechanism that resync all records when a new field is added has been implemented. This is made to ease the population of the values of these new fields in Marketo. I suspects this mechanism is responsible for this.
Now, in oder to avoid this, you should implement a sync filter (see Create a Custom Dynamics Sync Filter - Marketo Docs - Product Docs) and pay attention that, when you want to stop syncing a record, you need to do it from Marketo. See "Sync with Marketo" mysteries part 1 and "Sync with Marketo" mysteries part 2