Translation:There are many reasons that this happened.
"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...".