Only one default transition (outgoing) can be set for a given node. Add something like
If there's only one transition, set it as the default (e.g., from a state node or any transition in a linear workflow). If there are more than one transitions coming from a node, set only one to default. It appears as the first option in the UI, so it makes sense to make the default transition the one that leads to the best/most expected workflow path.