Similar thing happened to me. I was given a new title without a pay raise for starting and running a new national program in addition to continuing to manage my old team. I was literally working two distinct and demanding jobs at once. I was told there wasn't any money in the budget for a raise at the moment, and was basically told to be happy with a title change that wasn't even officially recognized in the corporate org chart.
After a very stressful year of getting the new program off the ground (while continuing to run my old team), I went to my boss for the umpteenth time and said that I needed: A. a raise; B. additional help running the new program; C. ideally, both, but I would settle for one of the two. I received D. none of the above, so I handed in one-month's notice.
The Director called me into his office and tried to get me to stay at a minimal increase, but I declined. A few weeks after I left, I learned that they'd had to recruit my old team leader back from another company to help run the new program, and they assigned my old team to another manager. They also hired a specific project manager to spearhead the new program. They had to create 2 new positions in addition to re-assigning my old team to a 3rd person to handle what I'd done by myself for a year.
Rather than accommodate me with what I considered a reasonable $20-30k raise, they ended up paying double that in insurance and benefits alone for the 2 new positions, not to mention their actual salaries. All in all, it probably costs them $150-$200k a year rather than the $20k it would have taken to keep me.
Now that I'm a little higher up in the corporate ladder at a different company, I've learned why they did that. Some larger companies are afraid of putting too many eggs in one basket. They actually would rather pay 2-3 people to do what 1 other person could do if it would mean that the 1 person would be able to set up a mini-fiefdom within the company in a business critical program. It's more power and influence than they want to give anyone who isn't a VP. It's pretty messed up, but that's how a lot of large companies operate.
TL;DR: Left old company. They had to replace me with 3 people. Big companies let this happen deliberately so that 1 employee is never too important. The lesson it teaches employees is that you're always a cog in the machine who can be replaced. Even a large important cog can be replaced with multiple smaller cogs.