Checked all xz commit timestamps for similar patterns. first is a series of commits by Jia Tan on Jan 19, then another Jan 22, then Lasse has a series on Feb 9, then a long series that includes the commits mentioned above, then 3 more series by Lasse on Feb 17 and Feb 29. This certainly seems unusual.
but, I do find similar things in git.git history, Junio has a workflow that results in that legitinately
This suggests to me that xz's git workflow changed in January.