The BBC has a news article saying: "Mystery Russian satellite's behaviour raises alarm in US". But the article doesn't say how or why the behavior is "abnormal".
And the linked transcript of Remarks on Recent Russian Space Activities of Concern merely states:
Mr. President, in October of last year the Russian Ministry of Defense deployed a space object they claimed was a “space apparatus inspector.” But its behavior on-orbit was inconsistent with anything seen before from on-orbit inspection or space situational awareness capabilities, including other Russian inspection satellite activities. We are concerned with what appears to be very abnormal behavior by a declared “space apparatus inspector.”...
So, what's so special/different about this (unnamed) satellite's orbit?