He was filter feeding as usual, but a chunk of shark got in the middle of a plankton cloud that titan was feeding on.
After he got to the chunk, he suddenly got hooked. (get it because fish)
Then he started scavenging, leaving his school of fellow Titanichthys.
After a while, he got sick of scavenging, plus there was barely any carrion left.
So he started going after small sea scorpions.
He loved the excitement he got from chasing down prey, this was was way better than his days of filter feeding.
After sea scorpions, he moved on to large fish.
Then to sharks.
And finally, to other placoderms.
And the Grand Devonian Feast begins.