Translation:There are many reasons that this happened.
No, it would be incomplete. But you could change the word order and use 'av' instead: "Dette skjedde av mange grunner."
obviously we can't use hvorfor here, because it's interrogative, so is 'til at' the convention for sentences like
There're many reasons why.... I don't know why.... etc?
"Det er mange grunner til at [subordinate clause]."
"Det er mange grunner til det/dette/hendelsen/krigen."
As you can see, the subordinate conjunction at is only needed if the first clause is followed by a substantive subordinate clause, rather than a single (pro)noun.
For "I don't know why..." you'd use "hvorfor" just as in English: "Jeg vet ikke hvorfor...".