This doesn't really fall under the category of a brownout, but it seems like the most common way that teams lost power on the field this year was due to power plugs coming loose. It happened to even the best teams this year. Make sure you tape in or secure power connectors for the radio and roboRIO. Make sure that battery lugs are tightened at the battery terminals, at the main breaker and PDP, and that your SB50 is plugged in solidly. Any high-resistance connection can cause excessive voltage drop when you pull a lot of current.
Your roboRIO will automatically shut off PWM outputs and disable CAN based motor controllers if it detects a brownout condition. This happens at 6.3V. If you want to avoid this, then you will need to decide on a way to monitor voltage, and scale back motor controller output (reduce current) under certain conditions.
Page 7 of this document describes the roboRIO brownout bahavior in detail.
https://decibel.ni.com/content/servl...ser+Manual.pdf