My cat got a urethral obstruction and he was completely fine - how to prevent from happening again? Not sure how it happened in the first place

My cat has been on Hills Science Prescription Urinary Care kibble for 7 years since I adopted him because he had urinary crystals. He’s never had any issues up until two days ago when we noticed that he hadn’t peed in two days, so we took him to the vet. Turns out his urethra was obstructed so they had to sedate him and find the obstruction, which they said was a “mucus film” - not sure what that means? He drinks lots of water, literally not a single thing changed in our routine, and he’s also easily able to reach and clean his nether area and often does. How did this happen? How do we prevent it from happening again?