Some models seem to be upgraded more often than others. How are these upgrade cycles determined? Enough new features? Competition? Profitability?
This really depends but if you pay attention to the cycle of upgrades between successive models in a series, the tendency is to upgrade slower at the higher end and fastest at the low-end. This is not an absolute rule but is generally applicable.
The common element is that all cameras made by companies seeking profitability. There is often a large cost in designing and developing electronics and so manufacturers get the most return as they keep selling the same models to cover the initial costs and therefore increase profit-per-unit over time. This is just as true of cameras as of other electronics such as gaming consoles and tablets.
They eventually have to stop selling a model as its performance gets surpassed by competition. It is obviously why competition is good for consumers. Of course, they are constantly designing new models because it takes years and they cannot start after the ones they are selling are surpassed.
Sometimes there is a new technology to give a significant advanced, but not always. In this case, it is often that components are reused and we have seen several offerings which were little more than cosmetic upgrades.
Until recently some companies renewed their entire line of compact cameras almost annually while DSLRs would be a longer cycle, with professional ones on 3 or 4 because they also sell much lower volume which means more time is needed to recover development costs.
It really depends on the camera maker. Most will be because of the new features (newer image processor, higher fps, etc), but some of them it's just to "make their products look newer" without anything worth mentioning (perhaps they have a release cycle every year). Take example of Canon EOS 700D vs 650D. I would think the 700D should be called 650.1D as it has very slight improvement.