OK, I will preface this post by saying that I am a US Soccer Referee, I know a few of the PSRA referees, and I was supportive of the organizing efforts of PSRA 5 years ago.
I am not surprised by this. MLS fans hate the referees, the MLS office hates the referees, MLS coaches hate the referees. So it is unsurprising that the MLS office is forcing PRO to screw the referees by ducking negotiations. There are likely, I don't know this for sure but it is educated guess, two things these negotiations are going to be about:
1. Referees are fed up with red cards being turned over on appeal with the consent of PRO officials despite the cards being a) confirmed by VAR, b) being confirmed as correct by assessors, and c) being confirmed correct by PRO. For a team to appeal a red card in MLS a three person panel consisting of person from the USSF, the CSA (Canada), and PRO must
unanimously agree that the call was incorrect.
The biggest case was
this red card Ted Unkel gave in 2017.
As noted by the PSRA in their response after the red card was overturned:
So despite telling the referee that the decision was correct and "Difficult" to make, they turned around and voted against him on the appeal. It's hard to effectively referee if the League is going to throw you under the bus despite them telling you did a good job.
2. Use of foreign referees. And I'm not talking about Canadians.
The PSRA, and frankly a lot of US Referees, are worried about MLS basically bringing in foreign referees to work the league in a fashion like the Saudi league is doing right now. Why should we worry about that? Look at who refereed the MLS Cup this year (and 2016): Alan Kelly, an Irish referee who "retired" to come over and work as an administrator for PRO. When MLS and PRO locked out the PSRA referees in 2014 and brought in replacement referees guess who was first in line to referee? Alan Kelly. When Kelly hit the threshold to be a PSRA member, and therefore be under the terms of the CBA, the PSRA filed a suit with NLRB against PRO because Kelly was receiving additional benefits:
Source.
MLS and PRO have already done it once and it's fairly obvious they want to do it again, otherwise they'd be negotiating a new contract fairly.