The history of science is that
every freaking time we look somewhere new, we find something new. It happens over, and over, and
over, and
over again. We have a really bad track record of predicting things in advance in new domains. The exceptions are leaping to your mind precisely because you've heard about them because they're the
exceptions.
Also, to date, zero of those things have been "aliens".
So rushing to declare the first instance of what was completely obviously a new class of objects as "aliens" because it didn't behave like what we expected is not rational, because we should expect that new things don't behave like we expect. The odds that the first one of these we detect is also the one from aliens is just not a good bet.
I'd bet a tidy sum of money that in 25 years it'll simply be common knowledge that these class of objects sometimes have those characteristics because of some characteristic special to them. Probably something to do with having a lot of things that turn to gasses and exert accelerations on the object because they were never blown off by the solar wind or something because of them being in deep space for millions of years. Might be most of them, might be a small-but-respectable fraction, but I bet in hindsight this is recorded in the history books right next to "pulsars are alien beacons!" and with the exact same tone of lightly sneering contempt we hold for that now. To which I can only say to the future, let the record show we did not all think it was aliens.