Several reasons. We all get tinfling in our fingers occasionally but if it stays or reoccurs frequently it is worth getting it checked out by your doc.
Many possibilities. This is not uncommon and may be due to positioning. If it persists and is always located in the same fingers, it would be worth having it evaluated by a physician. You need to note where it occurs, how often, for how long, what makes it better or worse...These are the questions you will be asked.
Many causes. Simple things like hyperventilation may cause tingling but other causes, some even life threatening can be a cause for numbness and tingling. A more detailed history is requires such as precipitating causes, associated symptoms, and a complete physical examination with possible diagnostic testing.