Significant declines in jumbo mortgage-backed security prices tracked by the PrimeX index in recent weeks were due to investor panic, not collateral fundamentals, according to industry analysts. The sell-off began earlier this month after Fitch Ratings downgraded a number of jumbo securities and warned of negative equity, prompting fears of strategic defaults. Prices on the usually steady PrimeX index fell by as much as 10.0 percent from the beginning of October. Some of the sub-indices even fell below par for the first time. ... [includes one data chart]