I'd agree that Sanders, and really just the nature of the primary system, is pulling Clinton left on certain headline issues and probably changing the way she has to present herself. She needed to placate the environmental and union elements with her stances on the pipeline and trade pact, for instance. But I also think she would always have had to tack left to gain the nomination, somebody would have tried to outflank her for the more liberal part of the party base.
And by most accounts her policy positions (which seem relatively
detailed in their description, regardless of whether one agrees with those actual details) have been
carefully crafted after wide consultation over an extensive period of time, towards balancing progressive ideals, pragmatic implementation and broader electoral viability (or at least trying to find a balance, and again ymmv on whether the final balance is appropriate). The latter two elements may be partially what you're referring to in her tendency towards the centre ground. But I don't really know how much the rise of Sanders has changed those platforms.
