The issue is that the second attackers have extra information and are able to formulate their strategy accordingly. As it stands on live now both teams have the same objective regardless of who goes first or second, that objective is FULL CAP the point. With the PTR changes they have created MANY different possible scenarios. The first attack still basically has to go for the full cap the entire time so their strategy is left unchanged whereas the second attack could be anything from "just touch the point uncontested" to "we also need to full cap" which gives them a lot more options and information to work with. It works the same way in regards to defenses as well. First defense has the luxury of using a traditional defense at the choke the entire time because they can NOT lose the game yet and they are aware the other side is going for full capture (nobody is so risk averse as to game plan for obtaining a tiny bit of cap as their win condition). The second defense can only be hurt by the extra information because anything less than a required full defense is going to have them playing in a more unorthodox manner and will get worse as the defense requirements increase up to the point where you are REQUIRED to have someone sitting on the point at all times. First defense will NEVER be placed in a situation where a player is REQUIRED to sit on cap all round. You already see this happen on Payload maps but to a MUCH lesser extent. Think of those games where one attack side barely moved the cart. Usually these teams resort to running "gimmick" or "unorthodox" team comps in a sort of Hail Mary last ditch effort of defense. This is where you see Sym, Torb and perhaps an extra DPS/Tank instead of that second healer because you know getting wiped once means GG. It's not nearly as bad in this situation because it's not a requirement to change your team comp/playstyle as you are still able to defend in a traditional manner on or near the payload but for capture points it will legitimately require you to change your play style and force you to spread your team out. You will not have the option of running a traditional "optimal" defense all of the time if you are second, this is something first defense never has to deal with.

TL;DR - This change will merely increase the Attackers Advantage that is already heavily present in this game but only in favor of the second attackers.