The short version is that Nadler tried to link the their subpoena for former White House counsel Don McGahn to a matter that was already before Howell--Nadler's attempt to subpoena secret grand jury information from the Team Mueller inquisition. Given Howell's know political predilections, Nadler thought it would be a good idea to have Howell also hear the McGahn case, rather than take a chance on the usual random assignment of judges. Howell said no: "connections between the two cases are too superficial and attenuated" to bypass random assignment. Nice try, Jerry.
Howell agreed with DoJ and rejected Nadler's arguments all up and down the line. For starters, the Team Mueller grand jury material is controlled by the Federal Rules of Criminal Procedure, while the McGahn subpoena is a civil matter. Howell pointed out that there were no issues of fact or law in common. Here's the interesting part:
The House Judiciary Committee claimed that the cases are related because they both tie into what they are now calling an “impeachment investigation” of Trump. Their complaint against McGahn calls him the "most important witness, other than the President, to the key events that are the focus of the Judiciary Committee’s investigation" into Russian interference in the 2016 presidential election.
The DOJ, however, argued that the term “related” refers to cases that have “common issues of fact” or stem from a “common event or transaction.” They claimed the committee “gets it backwards” because they are “trying to relate completely unrelated cases simply because it filed them in service of its overarching desire to bring various matters together in its investigation of the President.”
The existence of an “impeachment investigation” has also been called into question. House Judiciary Committee Chairman Rep. Jerrold Nadler, D-N.Y., has stated that “formal impeachment proceedings” are underway, but the committee’s ranking Republican Rep. Doug Collins, R-Ga., said that is impossible because the House never voted to approve such an investigation.
Several House Democrats have claimed that a vote is not necessary, either because the Constitution grants them the power to conduct impeachment investigations or because a recent expansion of committee powers allows it.