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When a traditional single-threaded application program is running, the statements making up the program are executed one at a time, in sequential order. Program control structures (loops and branches) repeat statements and may branch to alternative sections of code, but the important point is that, at any given instant, only one statement and one routine is being executed. This is true even of different instances of a program being run by two users at the same time (for example, a text editor). As each process is assigned a scheduled slice of CPU time, the statements are executed in the order maintained for that invocation of the program.
Drivers, however, are part of the kernel and must be ready to run as needed at the request of many processes. A driver may receive a request to write data to a disk while waiting for a previous request to complete. The driver code must be designed specifically to respond to numerous requests without being able to create a separate executable image for each request (as a text editor does). The driver does not create a new version of itself (and its data structures) for each process, so it must anticipate and handle contention problems that result from overlapping I/O requests.