There are generally 2 ways patents get infringed.

The first is some little guy comes to market, a competitor sees their product, and figures they can out-lawyer them in court. A great example is Fox pealess whistles. A variant of this is hiring experts who previously worked on patented or patentable material and having them copy the work without following reverse engineering practices in order to make sure you don't violate your previous employer's patents.

The second one is a patent troll gets an overly broad patent where anyone else doing work in the industry is going to infringe, mostly because the patent fails the obviousness test or there is prior art that the patent examiner isn't aware of. The SCO patents generally fall under this (as well as MS patenting Windows in the context of computers, Apple with their rounded rectangles, and others I can't bother to remember.

There are relatively rare cases where two inventors come up with the same idea to solve a problem (the flushing toilet is one) or someone reads filed patents and decides to copy the method directly, rather than obtaining the results using a different process.