不知怎么的，不知谁说的: for some reason, somebody (who knows who)
【高136】口语格式 / fixed patterns
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
不知怎么的, 不知谁说的: 不知 + question word + 的 is a parenthetical that admits the cause or source is unknown and moves straight on to what matters, stated after the comma. It is neither a real question nor an admission of ignorance for its own sake.
This page teaches the two clearest uses. 不知怎么的 ("for some reason, somehow"): the cause is unknown, often for a sudden change or a feeling: 不知怎么的，我一上台就紧张. 不知谁说的 ("somebody said, I don't know who"): the source is unknown, as with a rumour or hearsay: 不知谁说的，下个月要换排练厅了. In this second use 谁说的 is a full phrase ("who said it") and what follows is the reported content.
Do not confuse it with the full clause 不知道 + question word (我不知道他是谁), where ignorance is the message, or with 不知……好 (不知如何是好), where you are at a loss. Here the ignorance is set aside. 不知 is more formal than 不知道, yet 不知怎么的 is perfectly common in speech; you also hear 不知道怎么的, but the standard form, the one to write, is 不知怎么的.
Examples
不知怎么的，我今天特别想家。
Bùzhī zěnme de, wǒ jīntiān tèbié xiǎng jiā.For some reason, I'm especially homesick today.
不知怎么的，她说完那句话，屋子里一下子就安静了。
Bùzhī zěnme de, tā shuō wán nà jù huà, wūzi li yíxiàzi jiù ānjìng le.Somehow, the moment she finished that sentence the room went quiet.
不知怎么的，一听到这首歌，她的眼泪就止不住地往下流。
Bùzhī zěnme de, yì tīngdào zhè shǒu gē, tā de yǎnlèi jiù zhǐ bu zhù de wǎng xià liú.Somehow, whenever she hears this song she can't stop the tears.
不知谁说的，下个月公司要搬到新大楼去，现在大家都在议论。
Bùzhī shéi shuō de, xià ge yuè gōngsī yào bān dào xīn dàlóu qù, xiànzài dàjiā dōu zài yìlùn.Somebody said, nobody knows who, that the company is moving into a new building next month, and now everyone is talking about it.
不知谁说的，这家店的面条全城最好吃，结果门口天天排长队。
Bùzhī shéi shuō de, zhè jiā diàn de miàntiáo quán chéng zuì hǎochī, jiéguǒ ménkǒu tiāntiān pái cháng duì.Somebody, who knows who, said this shop's noodles are the best in town, and now there is a long queue at the door every day.
Common mistakes
✗ 不知谁说，下个月要搬家。 → ✓ 不知谁说的，下个月要搬家。 (谁说的 stays a full phrase: the 的 does not drop).
✗ 不知怎么的我今天特别想家。 → ✓ 不知怎么的，我今天特别想家。 (the parenthetical is set off by a comma, in speech by a short pause).
✗ 不知怎么的，我不知道为什么哭了。 → ✓ 不知怎么的，我哭了。 (不知怎么的 already says the cause is unknown: no need to say it twice).