[@Shiyonichi] Its the unique bond between Zubamon (A Legend Arms Digimon) and himself (A Tamer #Energy Supply for Digimon). Zubamon is a weapon and with every swing and bit of damage taken, energy is sapped from Phase. Unlike most Tamers who just supply energy to Digivolve, Phase is sapped for attacks, damage, Digivolution, all of that. And until he grows use to it and trains his body up even more. He'll continue to be exhausted, or in this case, go night-night. 

I figured the energy used to Digivolve, ontop of wielding Zubaeagermon, would waste him out. Not to mention being emotionally traumatize. Besides, this way I can show plot by expressing the emotions of Gilt and others while he's out.