All of a sudden a flurry of whispers echo through Castiel's brain, he freezes where he stands to try and concentrate on the thousands of voices all speaking at once in what seems to be different enochian frequencies. He realises what he's hearing is 'angel radio'. Without his grace it's difficult to understand and follow what is being said. Though as he listens more closely there is one repetitive aspect within the chatter. "DEAN WINCHESTER."
After a while of piecing together what is being said he comes to the conclusion that Dean has prayed to all the angels simultaneously, reaching out to anyone who, as Dean would say 'had their ears on' begging someone to help Sam in his worsening condition.
Castiel's heart sinks as he thinks of how many times Dean must have called out to him individually, not knowing that he couldn't be heard, desperately praying for his help. Needing him more than ever. And how in his desperation decided to reach out to any angel that would hear him. There's only one thing Castiel could do now. He had to make his way to Dean and Sam. Even though there was going to be God knows how many angels heading in that same direction. All of which want his head. But it's the right thing to do. Nothing felt more right to Cas in this moment than going to be with Dean, even if it may cost him his life.