Life-Threatening Situation ---Lost Steering Control on Highway

My 20 year old daughter was driving on a Texas highway at 70 mph (the allowed speed). Suddenly, the steering wheel began to vibrate and the car pulled hard to the right. She desperately tried to straighten out the car but it continued to veer right taking her completely off the highway onto the grass. She barely missed hitting a tree head on. After turning off the car and waiting to calm down, she started the car up and drove home at 50 mph without a problem. What could cause this terrible thing to happen?