Random location (uniform). 90% chance of a 2, 10% chance of a 4. This actually made it hard to model the computer move in the search.
When playing, it seemed like a 50% chance the new tile would be placed in exactly the spot or two that I didn't want it, even if there were 8+ open spaces.
That is a sampling bias caused by confirmation bias. Extreme cases such as the one pointed out tend to reinforce the hypothesis because they are more traumatic and, therefore, better remembered then the avalanche of cases where the higher probability positive outcome happens.