Alarm sounds when trying to remote start.

[Denethor]

Remote start has worked flawlessly for years. This week, anytime I try to use it, from any of the three fobs I have or the door keypad, it goes through the motions of remote starting (locks all the doors and I hear a click under the hood) then the alarm starts sounding. The truck is in ACC mode at this point because the stereo comes on, the dash lights up, and AC blower comes on, but the engine is not running.
In order to actually start the truck, i have to unlock the truck, get in, step on the brake to end the remote start session, lock, unlock, then put the key in and starts normally with the key.
Could the remote start module have become unregistered with the ECU or something? Makes no sense how it gets 90% of the way to starting the vehicle and then decides to sound the alarm.
Maybe a door sensor or something is faulty so it thinks someone opened a door during the start up sequence? I have no open door errors while driving or anything though, so that makes no sense either.
The only thing out of the ordinary is last week I came out to start drive home from work and it said the rear lift gate was ajar. I never use it, so I know I didn't leave it open, but assumed the button got pressed in my pocket or something. The same day however another car right next to mine at work, the trunk randomly popped open. Some sort of code scanner of someone trying to break in to cars?

 

[Denethor]

Turns out it was faulty battery, it was strong enough to start the car with the key, but something in the remote sequence saw too much of a voltage drop and would trigger the alarm. Strangest thing I've come across in a long time.