I wrote this a while back to another topic, haven't gotten anything new since that.
There is no straight true answer on what causes the muscle soreness. Some sources claim it's the lactic acid running through our muscles after a workout and during it, some blame inflammation, eccentric stress and muscle fiber damage. I think all of these have something to do with the soreness. However, this can also be the proof that it's not really the best indicator of progress.
Inflammation is a part of muscle regeneration and hypertrophy yes, but the most important things still happen on different levels. Resistance exercise causes rise in so many different processes, including protein synthesis, growth hormone, insulin sensitivity, glygocen synthase etc..Almost all kinds of resistance training will cause this. And coupled with correct nutrition, you get results.
Hypertrophy training tends to cause DOMS, atleast in the beginning of the program. It's purpouse is to fatique the muscle and give inadequate recovery during sets. Which will lead to lactate building and most likely more damaged muscle tissue. Time under tension will also trigger lots of hormonal responses and tend to activate some important pathways for growth.
But the longer you train, your body adapts to all that damage, and the DOMS reduce some amount. It happens with the beginners and the more intermediate trainers. I always have soreness when I start something new in the gym, no matter the rep range. Many people use BCAA's and anti-inflammatory supplements to reduce the soreness with success. That may incdicate that inflammation is a big part of DOMS.
Only thing to add is that eccentric-only training seems to cause DOMS more than concentric training. (That's why quads are burning on lunges, even though it's more of a hip movement than knee movement.) Maybe this is related to the muscle-damage and the loads the muscle has to endure while stretched.