I move very fast from companies that don't keep up with pay. I stay at companies with regular pay rises. I've had both. So in my mind it means that companies concerned with job hopping, are really hoping to keep your pay down. It's an early warning .
I accept people leave when their salary diverges more than a certain amount from the market rate but that would normally - even in a good market - take time.
Personally I wouldn't expect a major pay rise within the first 12 months of a job (if you do then you've probably not negotiated your starting salary properly). Add to that a couple of months after that to find another job and a month notice and the shortest stint relating to poor pay I'd expect is about 15 months. If you're staying 15 months or so in places which are bad payers and a few years in places which are good then your average is probably going to be 2-3 years. To me at least that's not a problematic amount of time (particularly not if you've shown you can stick at least place for 3 years or so because that says to me you're not inherently disloyal).
So short stints don't say anything, at least not much, about money when I read a CV.
Most employees are not loyal, because they know companies are not. If they think companies are loyal, they are probably new to the job market. Once a project ends, and there's nothing else to work on they will get rid of you. There's no reason why employees should not think the same way.
In the past five years a fair number of people have left the company I work for and I'd estimate about 25% were money related.
As I say there's no one rule but I've seen developers put up dreadful money for a job they liked and take a pay cut to get out of somewhere they hated and if I had to generalise I'd go with that over them following the pay cheque.
I've seen no data that suggests this is true, and from my own anecdotes I've found it to be untrue as well. The people I see that switch a lot seem to be simply unable to deal with colleagues or challenges that aren't going their way. Lots and lots of prima donna's in this industry that do not have the skill set to change things that are problems.
Even the money concern is a bit of a red herring. Being unable to justify pay increases at your current employer may be a sign of a bad employer, but it's just as likely the sign of someone who either cannot present their value proposition properly or someone who overestimates their value. Both are bad signs for the candidate.
Decent challenges, and decent colleagues are included in my definition of market price.
This turns out to not be true of lots of people. Being able to understand your value to your employer and to outline it in a compelling way is a skill that is very under developed in software tech candidates. It's also a skill that I've found correlates well with good hires.