It's doing this because the processors are humming along working hard. Nothing to worry about. That said, LR's performance isn't best in class...
Which version of Lightroom are you using?
What is your Operating system?
Do you see your Processor/memory usage shifting to very high in Activity moniitor when that happens
If so Post a screenshot of the same.
Also Post the System info from Lightroom>>Help
If you curious, you can obtain a temperature/fan control utility called 'Macs Fan Control' for free, which will show you the temperature at various locations around your MBP, and allow you to manually control the fan speed if you should so choose.
In my 2012 MBP, the processor temperature quickly goes beyond 100ºC when creating previews and during development tasks in Lightroom. It also quickly returns to a lower temperature as soon as the heavy lift processing is done.
The bottom line is, you can't do anything about the fans, it's kind of normal and indicates LR or whatever application you're using is in need of cooling because it's working hard.