pg_attribute has way too many row versions

pg_attribute has way too many row versions, how can I find out what's holding a reference to these row versions? I would expect at most 4-5K row versions in this table.

INFO: vacuuming "pg_catalog.pg_attribute"
INFO: index "pg_attribute_relid_attnam_index" now contains 15089905 row versions in 513390 pages
DETAIL: 724892 index row versions were removed.
29132 index pages have been deleted, 0 are currently reusable.
CPU 16.49s/8.57u sec elapsed 3799.71 sec.
INFO: index "pg_attribute_relid_attnum_index" now contains 15112894 row versions in 131544 pages
DETAIL: 724892 index row versions were removed.
200 index pages have been deleted, 0 are currently reusable.
CPU 4.03s/5.94u sec elapsed 964.61 sec.
INFO: "pg_attribute": removed 724892 row versions in 67005 pages
DETAIL: CPU 2.52s/3.30u sec elapsed 4136.68 sec.
INFO: "pg_attribute": found 724892 removable, 15000823 nonremovable row versions in 268172 pages

*DETAIL: 14998918 dead row versions cannot be removed yet.
*
There were 316065 unused item pointers.
0 pages are entirely empty.
CPU 27.13s/19.97u sec elapsed 9394.54 sec.
INFO: analyzing "pg_catalog.pg_attribute"
INFO: "pg_attribute": 268172 pages, 1877 rows sampled, 1877 estimated total rows