It is checked if at least one of the tags on the questions exist on the target site (possibly only as tag-synonym or intrinsic tag), if not the migration is automatically rejected. This can cause some unwarranted rejections, since one needs an exact match and already an extremely minor variation such as singular/plural can cause a rejection.
Especially related to MO there is the issue that on MO the main tag for statistics is named st.statistics, and not just statistics; due to MO using the math categories of arXiv as top-level tags, which are marked by a prefix.
Now, on this site (unsurprisingly) no tag st.statistics exists and while (I strongly assume) statistics would be an intrinsic tag here and a question thus tagged accepted, this is not the case for st.statistics and questions got rejected due to this.
The reason I raise this here, while not being active here but only on MO, is that if this should be resolved in a somewhat direct way it essentially can only be fixed here. I do realize that an initial reaction might be, why not just rename st.statistics to statistics on MO, but this would change the entire tagging system there, and synonyms on the source site do not help, very unfortunately IMO, to change this was the content of my meta.SO post mentioned above.
Two ideas I could offer to resolve this in a somewhat quick way (if this site even wants to receive questions from MO tagged only st.statistics):
Ask for st.statistics to become an intrinsic tag.
Make st.statistics a synonym of something fitting here, e.g., mathematical-statistics, on the grounds that if it comes from MO it should be mathematical-statistics (though in reality this might not always be true yet it might be better to have some resolvable mistags than rejections through-out).
Sorry for bursting into your meta with some suggestion/request, but I thought the issue might be of interest here. One could also envision to work on more compatibility of the tagging systems, but the migration traffic is not that high so maybe it is not worth the effort to work on details.
Of course there could also be issues in the converse direction, this site to MO, where the problem then is easier to fix on MO (as then synonyms there could suffice). If there should be such problems and you let me know, I could communicate and try to resolve them there. (I have no official role on that site, but can say to be pretty involved with tagging there.)