for a/some reason/reasons best known to himself, herself, etc.

for (a/some) reason(s) best known to (oneself)

For strange or perplexing reasons that the speaker cannot understand. For reasons best known to herself, Mom refuses to wear her glasses and goes around squinting at everything instead.
See also: known
Farlex Dictionary of Idioms.

for reasons best known to himself (or herself, etc.)

used when recounting someone's behaviour to suggest that it is puzzling or perverse. chiefly humorous
See also: himself, known, reason
Farlex Partner Idioms Dictionary

for a/some reason/reasons best known to himˈself, herˈself, etc.

(humorous) for a reason or reasons which other people find hard to understand: For reasons best known to himself, he wears two pairs of socks.
See also: known, reason
Farlex Partner Idioms Dictionary
See also:
  • for (a/some) reason(s) best known to (oneself)
  • for reasons best known to himself
  • for XYZ reasons
  • XYZ
  • have a soft spot for
  • have a soft spot for (someone or something)
  • have a soft spot for or an animal
  • have a soft spot for somebody/something
  • have a soft spot for someone/something
  • the short and the long of it