I don't believe that there is any direct canon answer to this question, so please be aware that my answer is purely speculative (within the spirit of canon).
The simplest answer is the old Voldy is shown throughout the series to be a character who needs to be in control, whether it be by pulling strings in the background (Pius Thicknesse becoming Minister upon the assassination of Rufus Srimgeour) or by directly controlling those around him (either through fear and intimidation or the imperious curse). That being said it is not that far fetched to suggest that Voldemort would be controlling to the point of assigning his death eaters specific places, or a particular order, to stand in upon being summoned.
Based on where a particular death eater is assigned to stand may indicate prominence and/or rank within the death eater order. For instance, in the beginning of the Deathly Hallows, Snape is afforded a seat of prominence, emphasis is my own:
"Severus, here," said Voldemort, indicating the seat on his immediate right." "Yaxley -- beside Dolohov."
The two men took their allotted places.
Harry Potter and the Deathly Hallows, page 3, US Hardcover Edition
As we can see in the italicized part of the quote, Snape is given the seat on Voldemort's right hand, which is typically regarded is a prominent spot, that would seem to have been saved particularly for Snape. Yaxley, on the other hand was assigned a seat elsewhere at the table. The boldface text shows that seats were indeed assigned to Death Eaters, and based on the preceding text, we can easily assume that it was Voldemort himself that assigned the seats, instead of say Malfoy who owned the manor.
Furthermore, it is not uncommon that in the military (or even militia armies) that soldiers assemble in a given order, generally by rank. As the death eater order is the closest thing we see to a trained army/militia in the Harry Potter series, it makes sense that they would operate and assemble in a similar manner.
And just to cover all of my bases here, before anyone can say that Voldermort would be completely ignorant of military history/formations, let's remember that for the first eleven years of his life, Tom Riddle was raised in a Muggle orphanage and presumably attended a muggle school during this time. I'm not so sure about the English education system, but here in the States, we do start learning a bit of military history, particularly WWI and WWII in primary school. A very basic knowledge of Muggle armies could have planted a seed in Tom Riddle's head, long before he became Voldemort, on how armies assemble, attack, defend, and ultimately achieve victory, or lose in battle.