As was said, this is normal behavior. Also, sometimes apps listed as running actually are doing nothing, they're only cached in memory.
Whatever you do, DO NOT use any task killer or memory optimizer. They actually waste power and may disrupt critical system processes and degrade performance. Android is excellent at handling processes and memory with no help from 3rd-party apps or user assistance. Just let Android do it's job, relax and enjoy your phone.
...for a full explanation.
Linux user #266351. Android since v1.0