According to the Mishno Bruro, this practice would likely be the result of an extralegal opinion that holds one takes six steps at the end of the Amido. (See קכ"ג סק"ח)
The Shulchon Oruch goes with the opinion that at the end of the Amido you take three steps back and you're done. That is, l'Halocho you do not need to take three steps forward. (See id.)
There is an opinion, not מעיקר הדין, that says no, part of the Amido is to take another three steps forward after you take three steps back. This is standard practice in many places though not necessarily as a matter of principle. According to this opinion, walking in front of someone who has not yet taken three steps forward is forbidden just like walking in front of someone who is in middle of the Amido, because this person who has not yet taken three steps forward still is in middle of the Amido. This is what the practice of not passing in front of someone who has not yet taken three steps forward is likely based on. (See id.)
All this is just an inference from something the Mishno Bruro says, because the Mishno Bruro is not actually talking about a practice to refrain from passing in front of people at that point in the Amido, but rather a practice of people standing in their place at that point in the Amido preventing others from passing in front of them. The Mishno Bruro understands their protest as based on this explanation. The Mishno Bruro does not specifically discuss whether refraining on one's own is a worthy practice, and one might argue that this is because he assumes the עיקר הדין governs in this context. One thing is for sure though, if a person is not protesting it is certainly not forbidden to pass in front according to the letter of the Halocho.