Israel has decided to encrypt the video data coming from its UAVs, to prevent Hezbollah and Hamas from looking in on the currently unencrypted video transmission. The U.S. decided to do this last year, after they found Islamic terrorists in Iraq using freely available hardware and software to eavesdrop. This capability was apparently supplied by Iran, which also provides help for Hezbollah and Hamas.
The Iranians had discovered that a $26 bit of software (SkyGrabber) enabled someone with a satellite dish and a laptop to intercept and capture video being sent to the ground. Normally, this is used by people to hijack Internet data being sent down via a satellite link. The idea for adapting this to grabbing Predator video transmissions did not require a lot of technical knowledge. In Iran, where satellite dishes are illegal, many people have responded by hiding small dishes, and doing whatever they can (often illegally) to obtain satellite based broadcasts. The electronic media in Iran is state controlled and censored by a bunch of Islamic conservative clerics. The Iranian secret police have thus come to learn all about stuff like SkyGrabber, and tipped off their brethren in the Quds organization (which takes care of support for foreign terrorists). U.S. troops discovered the terrorist use of SkyGrabber two years ago.
As for why the Predator video was liable to this kind of capture. Well, sometimes, you take a chance. And then you get caught. For years, the video from UAV was unencrypted. This was to save communications capacity ("bandwidth"), which was always in short supply. To encrypt the video would require more bandwidth, and specialized equipment on the UAVs and ground receivers. There would also be a slight delay for the guys on the ground using the video. For all these reasons, the video remained unencrypted. This was not a secret, it was known to people in the business. Now everyone knows, and encryption, and all its costs, will be added to UAV video broadcasts in more and more countries.