It is typical business, but **not good**, practice. Almost 10 years ago I’m sure your $5.99/mo was an awesome account for them. Now their focus is on agencies and companies that pay the 4-5 figures a month. They probably don’t really even want your $75/mo.
This large increase isn’t a cost of service adjustments (e.g. if they increased from $5.99 to $9.99/mo after 10 years), it is a we don’t want this user anymore. Why this is bad is because they are dissing the people who helped get them where they are. Without you and other $5.99/mo users all those years ago there would be no HootSuite.