Of course the bandwidth limits are hit due to dropping reserve rate (what else could it be, after all). The core question then is why exactly the reserve rate is dropping which you did not mention, so let me add a bit to your explanation.

Imagine the situation where there are a total of 1000 Steem users total each with 1 SP on their account. Suppose the total network bandwidth is 200 posts per day.

If everone wanted to post, each SP would be entitled to just 0.2 posts a day - this would correspond to a reserve rate of 1 and be overall quite inconvenient.

Suppose that initially just five users are posting two times a day, hence the network only sees 10 posts a day. This 20 times lower than its actual throughput, so the network adjusts the reserve rate to 20 and every SP becomes entitled to 0.2*20=4 posts a day. So far, so good, the limits are above average use for everyone.

Now, suppose that June 2017 comes and a horde of newcomers floods in. Let's say, 30 newcomers each with SP=0.05 join the club. A typical newcomer is eager to start posting daily right after registration and given the original rate it can do so (0.05 * 30 = 1.5 posts per day). However, as soon as 40 daily post attempts onto a 200 ppd bandwidth arrive, the network scales the reserve rate down from 20 to 200/40=5, which means each SP becomes entitled to just about 1 post a day - this is uncomfortable for the old users who needed 2 posts per day, and useless for the new ones, who now have a 0.2 post per day limit.

Note that nowhere along the way there was actually even a risk of going over the bandwidth - 40 posts per day would perfectly fit through the system, yet everyone still suffers.

Why? Because the posting intensity is, in reality, disproportional with the user's SP holdings, while in the bandwidth model it is.

If you think of it, this is a conceptual flaw in the system. Instead of allocating bandwidth preferentially to SP owners it, in fact, it should be given to active creators of valuable content, who are often different people. The rate limiting idea is meant to stop spam, yet it treats all content equally, and from this perspective a bunch of valuable posts from newcomers are indistinguishable from spam.

Instead, it should take the upvotes and reputation into account. Wouldn't it make sense if a whale with low upvote, no reputation and not much network use would be allocated less bandwidth than a valuable contributor, even if the latter has 0.01 SP.
After all, collaborative filtering can usually filter spam much better than capital requirements. On the contrary, the current system allows a whale to spam as much as they want, regardless of content quality.

In addition, the whole voting-bot and various lottery nonsense only makes things worse, as a lot of bandwidth is spent on non-content-related stuff.