The same resources can be accessed by several kernel processes in parallel, causing potential concurrency issues
Several user-space programs accessing the same device data or hardware. Several kernel processes could execute the same code on behalf of user processes running in parallel.
Multiprocessing: the same driver code can be running on another processor. This can also happen with single CPUs with hyperthreading.
Kernel preemption, interrupts: kernel code can be interrupted at any time (just a few exceptions), and the same data may be access by another process before the execution continues.