Silent Data Corruption is a wrong data modification on the drive, which comes from latent errors on hardware and software sides. On the hardware side, problem depends on drives run-out, head failure, noisy data transfer, power shutdowns and others. On the software layer, errors occurs due to coding bugs in OS, file system, devices firmware, and anywhere else where data exists in the computing stack.
The main problem is that on software side this corruptions are not detecting. System doesn't check data integrity if RAID has all drives on their own places. In this situation, system provides wrong data to application, and application, in turn, produces wrong results.