It’s converting null to the string "null" and trying to convert it. For radixes 0 through 23, there are no numerals it can convert, so it returns NaN. At 24, "n", the 14th letter, is added to the numeral system. At 31, "u", the 21st letter, is added and the entire string can be decoded. At 37 on there is no longer any valid numeral set that can be generated and NaN is returned.
js> parseInt(null, 36)
1112745
>>> reduce(lambda x, y: x * 36 + y, [(string.digits + string.lowercase).index(x) for x in 'null'])
1112745