"Travelling" is not wrong and "Travelling" vs "Traveling" is a "British English" vs "American English" thing as well-explained in the linked Wikipedia article:

The British English doubling is used for all inflections (-ed, -ing, -er, -est) and for the noun suffixes -er and -or. Therefore, British English usage is cancelled, counsellor, cruellest, labelled, modelling, quarrelled, signalling, traveller, and travelling. Americans typically use canceled, counselor, cruelest, labeled, modeling, quarreled, signaling, traveler, and traveling.

I think the linked article, "Why do some words have double consonants while others have only one?" seems to be a good starting point. You have to get yourself familiarized with all those examples.

It is important to note that "If the stress is on the first syllable, the word only gets one consonant + ING." in two-syllable words, for example, happening or entering. "Travelling" is an exception as there is no rule but has some exceptions.

"Travelling" is not wrong and "Travelling" vs "Traveling" is a "British English" vs "American English" thing as well-explained in the linked Wikipedia article:

The British English doubling is used for all inflections (-ed, -ing, -er, -est) and for the noun suffixes -er and -or. Therefore, British English usage is cancelled, counsellor, cruellest, labelled, modelling, quarrelled, signalling, traveller, and travelling. Americans typically use canceled, counselor, cruelest, labeled, modeling, quarreled, signaling, traveler, and traveling.

I think the linked article, "Why do some words have double consonants while others have only one?" seems to be a good starting point. You have to get yourself familiarized with all those examples.

It is important to note that, in two-syllable words such as happening or entering, etc.

If the stress is on the first syllable, the word gets only one consonant + -ing.

"Travelling" is an exception. There is no rule but has some exceptions.