Here's why:
* The vowel sound in "happen" is /æ/ (as in "cat"). This is considered a short vowel sound in some contexts, but it's not a "short vowel" in the sense of being a short, clipped sound.
* Short vowels are typically associated with a closed syllable (one that ends in a consonant). In "happen," the "e" at the end is silent, making the syllable open.
* Open syllables tend to have long vowel sounds. This is because the vowel is not followed by a consonant, allowing it to be pronounced more fully.
Therefore, while the sound in "happen" is similar to what is considered a "short vowel" in other words, it's technically a long vowel sound because of the open syllable structure.