〜せいか means "perhaps because" or "maybe because" — you''re not sure if X is the real reason, but it might be.
Uncertain causation - Maybe it''s because of X...
Possible explanation - Perhaps due to...
Perhaps because I''m tired, I can''t concentrate.
Less certain than せいで — more speculative.
Plain form + せいか
- 年のせいか (maybe because of age)
- 疲れているせいか (perhaps because I''m tired)
Expresses uncertainty — you''re guessing at the cause
Maybe because of the weather, I have a headache.
Perhaps because I'm nervous, my voice is shaking.
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