Why is the sequence, "1,3,2,4", not a bitonic sequence?
In this sequence, lets consider 3 as the bitonic point. So 1,3 makes for a strictly increasing series. 

However after we cross the bitonic point 3, 2 and 4 do not make a strictly decreasing series.

Another point in Bitonic Series' is that we have to take into account circular bitonic patterns. So a series like 1 2 5 0 -5 -2 is bitonic since we monotonically increase from 1 to 5, then monotonically decrease from 5 to -5 and again we monotonically increase from -2 to 5.

In the case of 1 *3* 2 *4* 1, there are 2 peaks occurring and hence its not even circularly bitonic.