After re-ordering, we can see that both 2 and 6 shows up 4 times, two more than the next numbers that have the next largest amount of numbers (1, 4 and 8, each having 2 numbers)
However, now you are adding another 6 into the set.
Explanation: Mode is the number or numbers that appear the most. Before you added an extra 6, the mode was 2 & 6 because there was 4 2's & 4 6's but when you added another 6, the mode changed to 6 because the 6 appeared 5 times.