Can Anyone Explain URL Permalink Changes?

For some reason the global URL permalinks of tags (and categories) on my website have changed recently. Could this due to recent WordPress updates? This is really messing up in-house ads on my website.

My permalinks are set and have been set, for example, for the past 3 years to:
http://www.bmedreport.com/archives/26523

Previously the tag URL structure, for example, was:
http://www.bmedreport.com/archives/tag/featured-meditation

But then automatically changed to (for unknown reasons):
http://www.bmedreport.com/tag/featured-meditation

And then recently reverted back to the original:
http://www.bmedreport.com/archives/tag/featured-meditation

Can anyone explain this phenomenon? All of these URL/permalink structure changes are reaking havoc on in-house ads and my Goals in Google Analytics. Could I be doing something wrong?

Thanks!
Chris