Need help understanding IC Behavior
From the datasheet:


Control-Logic Inputs
The control inputs (3/5, SHDN) are high-impedance
MOS gates protected against ESD damage by normally
reverse-biased clamp diodes. If these inputs are driven
from signal sources that exceed the main supply 
voltage, the diode current should be limited by a series
resistor (1MΩ suggested). The logic input threshold
level is the same (approximately 1V) in both 3.3V and
5V modes. Do not leave the control inputs floating.



Sounds like you left the control input floating, in violation of the explicit direction in the datasheet, which leaves the CMOS input in a high impedance state, susceptible to nearby electric fields (like those created by the charge stored on your finger). In essence, the behavior is unpredictable in this condition.