What it means
せいか expresses uncertainty about a cause: “perhaps because” or “possibly due to.” It links a suspected cause to an observed result, often one that is undesirable, unusual, or difficult to explain. The speaker presents the cause as a possibility rather than a definite fact.
Usage note
せいか follows a plain clause. After a noun, use のせいか, and after a na-adjective, use なせいか. Do not confuse it with せいで, which usually presents the cause as definite and often conveys blame or a negative consequence.
Example sentences
新しい薬を飲み始めたせいか、最近は夜中に何度も目が覚める。
Perhaps because I started taking a new medicine, I have been waking up several times during the night recently.
雨が続いたせいか、駅前の花屋にはいつもより客が少ない。
Perhaps because the rain has continued, there are fewer customers than usual at the flower shop in front of the station.