Taking three steps back into another person’s Shemoneh Esrei airspace, if he has already concluded Shemoneh Esrei:
The general rule:
It is forbidden to enter within four Amos [2 meters] space that is in front of a person who is in the midst of praying Shemoneh Esrei. Accordingly, even if one already finished his own Shemoneh Esrei, he may not take three steps back into the four Amos space of the person behind him, if that person is still in the midst of Shemoneh Esrei. Now, the question is asked if this likewise applies if the person in back of him has already finished Shemoneh Esrei but has yet to take three steps back, such as due to that there’s someone behind him who is still a middle of Shemoneh Esrei.
Some Poskim rule that the above law only applies if the person in back of him did not yet finish Shemoneh Esrei. If, however, the person behind him he has already completed his Shemoneh Esrei, then one may take three steps back into his four cubit space even if the person behind him has yet to take three steps back after his own Shemoneh Esrei. Other Poskim, however, rule that it is forbidden to take three steps back into the four cubit space of a person Davening in back of him until that person also takes three steps back. Practically, the widespread custom is like the former opinion.
 Admur 102:4; Michaber 102:4; Rebbe Yehoshua Ben Levi Brachos 27a
The reason: As this may cause him to become distracted. [Admur ibid; M”A 102:6] Alternatively, the reason is because during Shemoneh Esrei one is standing before the Shechina, and it is improper for people to trespass this area. [M”B 102:15 in name of Chayeh Adam; Admur 102:1 regarding sitting within four Amos of the Shechina and Ketzos Hashulchan 20 footnote 26 that the same applies regarding walking in his space]
 Admur 102:5; Michaber 102:5; Orchos Chaim Tefila 6; Mahariy Abuhav; Yerushalmi
Other opinions: Some Poskim rule that it is permitted to take three steps back into the four cubits of a person Davening in back of him. [M”A 102:7 in name of Rashi]
 See Piskeiy Teshuvos 102:1 and 4
 Implication of Admur 102:4-5 [who writes the reason for the prohibition is that it will cause him distraction in prayer]; Betzel Hachochma 6:32 [based on M”A 422:2] that so is custom; Az Nidbaru 3:45; 6:47; Halichos Shlomo 8:34 that so is custom; Or Letziyon 2:7-27; Sefer Birur Halacha 102; Piskeiy Teshuvos 102
 The reason: As the main issue is that the person behind it will become distracted from his prayers, and hence if he already completed his prayers there is no distraction that will occur to make it for forbidden. [Poskim ibid]
 M”B 102:3 in name of Shulchan Shlomo regarding sitting and that the same applies regarding walking into his area; Kaf Hachaim 102:21; Ketzos Hashulchan 20 footnote 26 that so is implied from Admur 102:1; Halichos Shlomo ibid that it is best to be stringent
 The reason: The reason for this is because the Shechina is found in front of the person until one takes three steps back [See Admur 97:2; 102:1; 123:1; M”B 122:5; 97:3] And this is the entire reason for why one may not sit within 4 cubits of a person still in middle of his prayers [see Admur 102:1] and the same should apply regarding taking three steps back.
 Betzel Hachochma ibid; Piskeiy Teshuvos ibid