I'll have to start answering your question with another question. What do you mean "all of a sudden"? The UFT has been playing dead for as long as I can remember. If you've been around long enough, you can remember when starving artists made more than teachers. While that has improved somewhat, we have given up all our rights in exchange. The 2005 contract is a prime example of the UFT rolling over for the city. I won't recall all the horrors of that document, but judging by the way things are going, I wouldn't be surprised if five years from now teachers think of the 2005 contract as the "good old days" of teacher unionism.
Also, the UFT is an old dog and it's hard to teach them new tricks. In fact, they've been in power so long that a more accurate metaphor is that they are an old, fat, lazy dog. They are more than willing to wallow on their backs and get their stomachs rubbed by the press, but just try asking them to get up and fetch you something useful, like a raise or a guarantee that you won't be laid off after 25 years of service. That dog won't hunt.
Money has added to the fat lazy dog syndrome. The UFT has just about zero incentive to stick up for their members these days. They get their $1000 a year from each of us no matter what. They want hard working teachers who get rated 'ineffective' because they teach low performing kids to get the boot. Maybe things would be different if the UFT had the same system. If the union bosses got booted after two years of being rated 'ineffective' by the members maybe they'd have to actually do something for a change.
As to their silence in the face of all the negative ads, the answer is simple. Those ads are funded by hedge fund folks who view public education as their own personal playground. For the UFT to fight them, they'd have to spend money on their own ads and PR people, and that costs money that needs to go to pay for the second pensions of union honchos and all those nice conferences they get to attend instead of actually teaching.
The UFT may be comprised of a lot of fat lazy dogs, but they're not stupid fat lazy dogs.