An altimeter brown-out can be caused when when a high current load drops the battery voltage below the point where the microprocessor can reliably operate.  If that happens the microprocessors program can operate erratically or randomly jump around.  A failure to deploy or a premature deployment is usually the result.

The Marsa54L microprocessor runs on 5v but can reliably run as low as 1.8v. However the barometric sensor and accelerometer are not rated for accuracy below 5v.  So there are two limits that need to be managed, the 5v one to maintain accuracy and 1.8v to avoid complete shutdown.

Weak Battery Protection

If the battery voltage drops below 7V the Marsa54L will not beep during the waiting for launch mode. If you use the LED indicator and orient it so you can see it from the flight line you can monitor the go a distance.

High Current Battery Voltage Drop Protection

Batteries with moderate internal resistance like alkaline or rechargeable NiHM will drop voltage if a high current load is drawn. A possible failure mode exists when altimeters are used for airstarts. When an igniter is in the combustion chamber of an ignited rocket motor the hot combustion gases can present an almost dead short.  This can cause the battery voltage to drop low enough to reset the altimeter.  To prevent this failure mode the Marsa54L monitors the battery voltage during a deployment event.  If the battery voltage drops too low the Marsa54L will reduce the current to the outputs to keep the battery voltage at a safe level.