The recent collision between a British and French nuclear submarine is not as rare as most people think. Four years ago a U.S. SSN (nuclear attack sub) collided with an underwater mountain. The sub survived, but its sonar dome was smashed in, and one sailor died.
The Golden Age of submarine collisions was during the Cold War (1948-91). Once Russia began building nuclear subs in the 1960s, and putting them to sea often and for long periods, there were lots of collisions. Well, about one every two years. Most involved at least one Russian boat. The problem was that the Russians had pretty poor sonar, so they were the equivalent of half blind under water. From the 1970s on, the U.S. has increasingly superior sonar compared to the Russians. This led to the more collisions involving Russian and U.S. boats. It also saw the invention, by the Russians, of the "Crazy Ivan" maneuver. This occurred when an American sub was stalking a Russian one (often an American SSN keeping tabs on a Russian SSBN). The U.S. boat would stay in the Russian subs "blind spot" (behind its propeller). But sometimes the Russians would suspect they were being stalked, or just wanted to make sure they were not, and would perform the "Crazy Ivan" maneuver, which involved upping speed and making a sharp turn. The U.S. sub would have to quickly get out of the way, or there would be, and sometimes was, a collision.
Most of the collisions during this period involved Russian subs bumping into other Russian subs, or inanimate objects (icebergs, oil rigs). Western boats had far fewer collisions because they had better sonar, and better trained and more experienced crews.