Tell us the all of the details of your computer hardware. Also please tell us the camera that took these photos, and are they RAW or JPG.
iMac (Retina 5K, 27 inch, Late 2015)
Processor 3.2 Ghz Intel Core i5
Memory 8GB 1867 Mhz DDR3
Startup Disk Macintosh HD
Graphics AMD Radeon R9 M390 2048 MB
Images are RAW
I've also had this issue when exporting using two presets at once since this update came out (used to be able to do 4 export presets at one time... older versions of lightroom could handle 6 export presets at one time). The latest version of LR for Mac is so carelessly put together and my buddies on PC claim that the latest version for them uses so much RAM if the program is left open for awhile that it requires a hard restart. Each time it's crashed for me, I've sent in the crash report and it has crashed at crucial times when under extreme deadlines.
For reference, the files I'm using are RAW from dual 1DX's and 5DIII. Computer is mid-2014 15" MBP 2.8QC i7/16GB/1TB SSD w/ Nvidia GT750M 2GB GPU