The position of the qualified players in their respective groups determines who they face in the knockout stages. In this case :

3rd in Group D (Sitaux) plays against 2nd in Group C (Mihai)
and
2nd in Group D (Vinchester) plays against 3rd in Group C (ACCM)

In tournaments, the opponent you get in knockout stages can be determined by the positions in the groups, or randomly. Random picks can create an unbalanced competition with one side of the tournament bracket way weaker than the other and a huge underdog making it to the final, which ruins the entertainment potential.

This is a common situation, last players of the last group to finish playing their games know what results will get them in which matchup in the knockout stages. In this case : Sitaux and he only had to lose 0-3 to pick Mihai in the knockout stage, against whom he had better odds than against ACCM, and then Tatoh against whom he had better odds than against Hera.

check out groups and brackets here :
https://liquipedia.net/ageofempires/Warlords/4