right this should be in spoiler section but i'll give the answer u need.
It's speculated that the hollow form comes out in times when Ichigo borders death and is about to die. Thus it is almost a survival instinct. If u've seen the Ichigo vs. Zaraki fight where Ichigo goes into his inner world, there appears a white figure who are dnt know who he is. Wut's more is tht he's part of zangetsu and ichigo as well. We can only assume it is jus a part of survival instinct.
The personality taken on when the hollow mask form is the white figure in Ichigo's inner world