That a sovereign decision was taken on NATO membership is not debatable, it is the truth, but not the whole truth.

Parts of the truth are that the expansion also underscored the inter-alliance issues (American role in Europe e.g.), it was also a product of internal American political debates, a product of European inability to create more robust security institutions, a product of bureaucracy perpetuation,  etc.

Another part of the truth is that after '91 there was no communist dictatorship nor SU. So, the logical answer to the question 'why join NATO?' was fear of Russia, whether rational or irrational. Thus the expansion of NATO through East European members got unmistakable anti-Russian flavor.

Since alliances, at the end, do not really exist without answering the 'who is our enemy' question, we arrive at the logical conclusion that NATO's 'expansion' to East was indeed anti-Russian step in its essence. If it had no internal content in that matter shortly after the end of the Cold War, it soon got one, conciously or unconciously.