I don't see where the Legion had such a habit. They only did it twice; and both times, it was after some petty noble tried to pin atrocities on them, murder them, and steal their landhold. Exactly how much abuse are mercenaries expected to take from their employers before the MRBC deems it acceptable to engage in self-defence?
Mercenary's Star: He took the Legion into direct combat with Combine forces in violation of his contract with the Free Verthandi movement. While this was acting to save the rebel forces from destruction, it was in direct violation of the contract, which specified the Legion was there for training of the Free Verthandi forces only, with allowance only for self defense. (and given that defending the Phobos's crash-landing site got him in hot water with his employers, the self defense clause was apparently not all that strong.) while the Free Verthandi rebels didn't have a chance to report him to Comstar and Galatea's mercenary board, and later events would cause the people of Verthandi to ignore what had happened, when the details of the campaign got out to the wider mercenary market, you can bet employer's took notice that the Legion went against contract and did so without even the nicety of being able to claim they were following their employer's orders.
Price of Glory: After the unit was accused of warcrimes, rather than report to their employer's as ordered, the legion diverted to their landhold and conducted a running engagement with forces of their employer's. that the warcrimes were false and the forces in question were in the middle of conducting illegal warcrimes of their own is not really as big of an issue as the fact the legion once again directly violated orders from their employer's and in so doing, violated their contract. This is especially important given that while the legion was official cleared of all charges for the warcrime, the exact events as to how they were framed, by who, and the finer details of the events on Helm were not made public, so the majority of the inner Sphere is going to perceive them as "the legion was accused of warcrimes, and then instead of reporting to the capitol for investigation and trial, they went to Helm and shot up their own allies. but they found a star league cache, so the FWL swept it under the rug"
Tactics of Duty: Governor Wilmarth of Caledonia hired the Legion to assist in defusing a rebellion. the Legion refuses, and takes down Wilmarth's forces, then engages Lyran regulars who had also deployed to stop the rebellion. that the Legion had been ordered to commit warcrimes in order to uphold a governor who was guilty of further warcrimes, and then fought a skye separatist force that had been fomenting the rebellion in the first place, would not be a widely spread fact.
Operation Excalibur: the Legion attacks and forces lyran regulars who had attacked Glengarry in retaliation for the events on Caledonia. the Legion subsequently is stripped of landhold for violation of contract. they then get hired to help defend hesperus II, but then turns on his employer. that his employer's were skye seperatists wouldn't get out until much later (and the full details were still classified by the LAAF), and the fact that The Legion seeking that employment in the first place was due to orders from kathrine Steiner was top secret. so as far as the Inner Sphere is concerned, the Legion just hired onto and then betrayed three employers in rapid succession (the Fedcom, Governor Wilmarth, and Lord Gareth.)
the only book where the Legion doesn't break contract was
Day of Heroes. can't even claim Decision at thunder Rift, because greyson signs on with the Trellwan Lancers (the local governor's personal unit) to fight the pirates that had destroyed Greyson's parent's unit, but betrayed the Lancers when the DCMS showed up, defeated the pirates, and the local governor took in with Duke Ricol..
so yeah, the Grey Death Legion's reputation for breaking contracts is rightly earned, as far as the public record is concerned. the details that justify those contract breaks are stuff we know from the books, but in setting are either not well known, or classified.