Contractors and Permies sometimes are in the same boat
OK, whilst I don't think that these tactics are particularly "gentlemanly", contractors are in business and sometimes business is just war without guns.
Not nice, not very palatable, but it's the real world, deal with it. Over the last 15 years two of the three times this has happened to my business (not at UBS), I accepted it, the third I chose not to and the agency/client backed down within hours. This course of action may not be appropriate for all contractors, but they can and should resist if possible - that's just good business practice. They should also consider demanding that their agency take a lesser margin and share the loss - after all the large proportion of an agency's work is paid for within the first few weeks of a contract and with a reasonable allowance for admin/factoring costs the remainder of the contract is at a healthy profit.
If ALL the banks/agencies do it simultaneously then that smacks of collusion/cartel and the MMC, DBERR (or whatever they're called this week) might get to hear of it from a bunch of contractors with good lawyers, but on a bank-by-bank basis, this is just business and you can bet your bottom dollar that other suppliers are getting similar sort of pain.
Bear in mind that most senior employees of UBS are pretty much in the same boat with ever-decreasing (and paid in shares) bonuses, salaries held frozen for years at a time (not even CPI/RPI increases &etc) and ever-decreasing levels of morale.
At some levels old-hand staff in many companies are line managing recent joiners who are on much, much higher base salaries because the old-timers have had years of frozen pay and the newbies join at market rate or better. Old-timers may be reluctant to move because their bonus funds are in shares which are not yet vested so they feel locked in to their current employer (the obvious intention of such schemes).
BTW whilst it's fashionable to beat the banking fat-cats about the head, remember that for every barrow-boy trader or investment rocket scientist there a dozens and dozens of staff on "normal" rates of pay.