On June 17th, a Russian Su-24 bomber crashed while landing. The Russian Air Force did not ground all aircraft of this type, until an investigation to determine if there was a problem with all Su-24s. Two days later, another Su-24 crashed while trying to land. At that point, all Su-24s were grounded pending investigation results.
Less than two years ago, the Russian Air Force began receiving twelve upgraded Su-24 fighter-bombers. The improvements included modern electronics (all digital, flat panel screens, GPS and so on). The aircraft were also equipped to deliver all the latest smart bombs. Russia wants to get the most out of the few flyable Su-24s it still has in service. Three times that year (2007), all Russian Su-24 bombers were grounded because one of them had crashed. Su-24s have been in service for 34 years, and less than a third, of the 1,400 manufactured, are still technically in service. But, like the U.S. F-15Cs (about the same age as the Su-24), older aircraft get cranky and unreliable.
The Su24 was something of an "F-111 Lite" when it first showed up during the Cold War. The 43 ton, swing-wing bomber has a crew of two and can carry up to eight tons of weapons. The aircraft has inefficient engines, and lots of 1980s vintage electronics. When everything worked, the Su-24 was an all-weather bomber capable of delivering dumb bombs quite accurately. Most of the time, everything didn't work. The new upgrade (the Su-24M2) hoped to fix that. It sort of did, but most of the Su-24 fleet is simply too old.