If resetting preferences doesn't help, it may be that the fans are clogged or not working, causing the computer to overheat and abruptly shut down to avoid damage. This is a common problem with Lightroom on Intel Macbooks -- Lightroom uses computers much more heavily than most apps, thus causing the CPU/GPU to use more power, thus generating more heat.
To quickly test this, install the free Intel Power Gadget and monitor the temperature as you use Lightroom. If it's getting up close to 100, you've got a problem. Best to get it serviced.