What can cause Mom (84) to start cussing and yelling at people?

My mother has started using profanity and losing her temper with anyone who can't immediately answer questions or provide help when she feels she should get it. Her overall health is good (high b/p, well-controlled diabetes), she still drives and lives alone. While no immediate family is nearby, she has a strong network of friends with whom she's in contact daily. My sister and I have noticed this change in the last year.