"undefined behavior"?

Wrote a C program to mimic the stats shown on:
https://www.calculatorsoup.com/calculators/statistics/descriptivestatistics.php
My code compiles and works fine - every stat matches - except for one anomaly: when using a dataset of consecutive numbers 1 to N, all values  > 40 are flagged as outliers.  Up to 40, no problem.  Random numbers dataset of any size: no problem.
And values 41+ definitely don't meet the conditions for outliers (using the IQR * 1.5 rule).
Very strange.
Edit: I just noticed I didn't initialize a char:
before: char outliers[100];
after : char outliers[100] = "";
And the problem went away.  Reset it to before and problem came back.
Makes no sense.  What could cause the program to go FUBAR at data point 41+ only when the dataset is consecutive numbers?
Also, why doesn't gcc just do you a solid and initialize to "" for you?