One-Years??

You've flaired yourself here as "principal" so I'll assume you're employed somewhere, nevertheless I'll respond for the masses.

There's no industry-wide trend or conspiracy here. You're zoomed in on a particular instrument at a particular time when there happen to be lots of vacancies which cannot all presently be filled for the long term. Orchestras have not decided as a whole that it's better to hire musicians for one year at a time. They operate independently under the terms of their unique CBAs and under the watchful eyes of their local unions. Yes, the union is a national organization, but the locals are overseen by... locals. Sure, the ICSOM and ROPA conferences share data and strategies, but those are for the protection of musicians and preservation of our industry. I've worked for 7 salaried US orchestras either as a contract member or a sub, and each had completely different hiring practices and roster requirements. Yes, the League of American Orchestras gives administrators a platform to share ideas, some good, some bad, but ultimately the hiring process is strictly defined in each group's CBA and cannot be affected purely by the will of management.

As a matter of pure opinion, I don't believe any major orchestra in America would generally prefer to hire a one-year vs a tenure-track player. Sometimes, however, it's the best, only, or most necessary solution to a problem.

Some of those problems have been described below. Perhaps someone is on a leave of absence, a sabbatical, or disability, leaving a temporary vacancy. Perhaps someone won a job in a new orchestra and the contract requires their chair be preserved for a year in the event they are not tenured elsewhere, or perhaps the music director has elected to save that chair by choice. Perhaps someone moved up from a section chair to a title chair and left a temporary vacancy while they complete their probationary contract in the new chair. Perhaps there are numerous vacancies to fill and the orchestra does not have the resources, budgetary or otherwise, to fill all of those chairs with tenure-track players at once. Perhaps management reserves the right to delay hiring pending the resolution of a financial condition. Perhaps there is not presently a music director and one is required by contract to make tenure decisions. Perhaps someone won a job and subsequently declined to take the contract, leaving a vacancy with short notice. Perhaps there are a wealth of conflicting auditions nationwide for the same instrument at the same time, and an orchestra is not confident they'll have the opportunity to hear every interested party. Perhaps the orchestra is in a bargaining period and/or facing cuts.

The point is, there are innumerable possible explanations unique to each gig. You said this "trend" isn't good for musicians? I think it could be a lot worse. Those groups could be hiring subs by recommendation or at random with no equity or opportunity for those withou the right connections. Instead someone gets to win a year of job stability and income while someone else gets a year to recover from a surgical procedure, earn tenure in a new place, decide whether they're really ready to retire, further their education, take care of their newborn, etc... Someone gets a wealth of job experience beyond what they could hope for as an occasional sub. Someone gets to occupy a chair that is not fully vacant and to work alongside musicians they may hope to join in a permanent capacity should that chair become available for the long-term.

I don't think everything is fair and equitable in this industry, and I don't think every audition is run entirely without bias, but we can't always cry foul and point fingers when something happens that we are not entirely informed about.