Couple of things could be at play here.
What is your humminbird set at to identify low voltage? My humminbird low battery warning is adjustable. I determine at what voltage it comes on. What is yours set at?

Also, when starting a motor you are putting a huge initial draw on the battery. Hench a cranking battery verse a deep cycle battery for starting. Sounds like that draw is sucking the battery below your low volatge warning.

Depending on your voltage drop, this could also be an early indicator that your battery is losing a cell, or is low on acid, or needs a good charge. get the battery load tested. Most auto parts stores do it for free.