The New York Times reports what many on the right suspected — Schiff got a heads-up about the whistleblower complaint, indeed knew about it before it was filed. This affected how he handled the matter, which got the ball rolling on what ultimately became the impeachment inquiry:

By the time the whistle-blower filed his complaint, Mr. Schiff and his staff knew at least vaguely what it contained.Mr. Schiff released a letter seeking the complaint and suggested it could involve Mr. Trump or others in his administration. Mr. Schiff followed up by subpoenaing Mr. Maguire to testify before the intelligence committee.

Mr. Schiff’s intense push took Mr. Maguire and his aides by surprise, current and former intelligence officials said. In other cases of lawmakers seeking classified material that the intelligence agencies were reluctant to share, including whistle-blower complaints, both sides usually tried to resolve the matter by holding quiet discussions.

Officials in Mr. Maguire’s office, who did not know the details of the complaint, were puzzled why Mr. Schiff went public right away, eschewing the usual closed-door negotiations.