Variable Importance in unsupervised anomaly detection algorithms
There have been workshops dedicated to "outlier detection and description" (ODD), but there came out nothing from them that convinced me, unfortunately. but YMMV.
It definitely won't be enough to just use some library! You'll need to read and implement papers. There are subspace outlier detectors and correlation outliers, for example, that will tell you which features were relevant for a particular outlier.

But in general I believe you'll quickly run into multiple testing problems: in real data, every point is anomalous if you just try attribute combinations hard enough. Donald Trump is anomalous because of his orange skin, fake hair, and small hands, for example. If you only look at his xenophobia, he probably is pretty normal, unfortunately.