Why does my LN2 evaporate so quickly?
Your 4 observations:

Quick evaporation first showed up when finely divided particles entered into the situation
The finer the frozen food was crushed the faster the evaporation
Not grinding particularly aggressively (i.e. probably friction-heat from grinding not generated faster as the experiment progressed)
Toward the end of the experiment evaporation completed before any grinding even took place

sound like your speculation: &quot;Maybe something to do with the effective surface area increasing as the frozen food is ground into smaller and smaller pieces?&quot; is on the right track.
The particles of frozen banana &amp; peanut butter provide lots of places for the boiling of the LN2 to occur, just like if you dump a bunch of salt into almost-boiling water, it suddenly boils up like crazy where the grains hit the water.