Both @kingsley.tracy and @n6zs are wrong.
It has nothing to do with pronunciation collision.
The reason we use "ce" instead of "ces" is that "ces" is a demonstrative adjective meaning "these/those", while "ce" can be both a demonstrative pronoun meaning "they/these/those" and a demonstrative adjective meaning "this/that".
You may want to learn more about demonstrative pronouns and demonstrative adjectives.
Any time you want to write "Ces sont", just be aware that it is never written that way. To avoid a collision between the two "s" sounds, it is always changed to "Ce sont". If you hear "sont" in this context, you can be sure the preceding word is "ce". It means exactly the same thing, "These are...."
[Edit: Pardon me for posting the apparently incorrect information about colliding "s" sounds. It sounded reasonable when I first heard that. At any rate, according to Lawless, "c'est becomes ce sont when followed by a plural noun".] -31/08/15
It is more idiomatic, I think. The literal translation of "strong moments" is ineffective. Reverso's translation in context gives "highlights" as the most common translation of "moments forts". It refers to the most memorable features or moments which are being recalled, i.e., the high points or highlights. It doesn't mean that are hard or difficult, just memorable. http://dictionnaire.reverso.net/francais-anglais/moments%20forts
"Ce" is an indefinite demonstrative pronoun here which can be singular or plural. "ces" is a demonstrative adjective only used to modify a noun that is plural. The confusion comes from the demonstrative masculine singular adjective which is "ce" also, but which would always precede a noun. http://french.about.com/od/grammar/a/indefinite-demonstrative-pronoun.htm http://french.about.com/od/grammar/a/adjectives_demonstrative.htm
If you look on Reverso, you will find that "highlights" is the most common translation of the term "des moments forts" as it is used in context. http://dictionnaire.reverso.net/francais-anglais/moments%20forts