Many Christians see the Q Source/Quelle document as a "threat" to the authenticity of the Bible, as the claim is that the Gospels were written, being sourced from the Quelle.
So, many Christians seek to "debunk" it.
However, I have held a pet theory concerning the Quelle since learning about it's existence well over two decades ago.
When voting to replace Judas, the Apostles agreed to four criteria/qualifications that the new apostle must meet.
The new apostle had to have been with them from the beginning, seen Christ perform miracles, seen him crucified, and seen him resurrected.
I believe they picked Mathias because he was part of the throng of people who were following the trek of Christ and the Apostles from the beginning. But being that he was not directly involved in the events that transpired, Mathias was taking notes and documenting in a journal what took place.
The Apostles would naturally choose Mathias, because he had written down much of what transpired. This would explain why some of the accounts change from second person, to first person narrative.
Two cents.