About tomorrow's witness. Speculation is they are being shielded from right wing pressure.

[Stinky The Clown]

If true, it is a reasonable conclusion the witness is a well placed insider.

We shall see soon enough.

[Novara]

1. Could be. At any rate, this person is somehow at risk so that they had to have an immediate hearing

That implies they are vulnerable the longer it takes to get them into a hearing.

From the Washington Post:

It was not immediately clear whom the committee plans to interview and what topics lawmakers intend to focus on. The decision announced on Monday was shrouded in secrecy, with staff and committee members explicitly asked to go dark and not speak with media, according to two people involved with the investigation.

The news was so closely guarded that even some senior committee staff and aides to lawmakers were kept out of the loop as of Monday afternoon. Three people familiar with the investigation said that the secrecy is in part due to credible security threats to a witness.

There's more speculation in the link.

I'd rather not speculate. It could be a number of people.