One scenario I can offer, is that a publication sometimes goes rotten from the inside out - by the time readers detect a change in content, the real problem has been ongoing for some.
If you feel appreciated by a company, you'll work long hours on the title, and invest a lot of energy and care in creating its content, because it's a matter of pride, and a desire to communicate something worthwhile to the readers that they will appreciate.
It's certainly not for the money because you do not get paid a great deal, not in comparison to your friends who are quantity surveyors and environmental scientists.
But companies being companies, they will try to take advantage of their workers - for example, the unpaid overtime people put in for the love of the job becomes expected of you, rather than something extra. Add together all shorts of cutbacks or abuse, and resentment grows between employee and employer at the expense of the title, because by then, who wants to invest their heart in it? Who wants to work the extra hours to create special content?
And then readers start seeing a title that no-one cares for anymore, because creating a good magazine or newspaper isn't a 9-5 job. Editing - visualising, organising, guiding - requires you to bring into play your business brain, your creative side, and your inner drill sargeant, it's quite a personal investment. When the editor/top people stop investing themselves, the magazine starts drifting.
Then editorial staff leave. The cure is a new team, a new attitude and a new lease of life.
I've seen other publications die a slow death from mishandling of advertising sales, where the revenue slowly dried up, both from economic conditions and bad handling of clients. It's in the nature of salespeople to go for short-term results (bigger commission for them at the end of the month) without having the wit to plan ahead for the year. A good salesperson would - but they're outweighed by the shysters who got the job because they were good at talking ****e down the phone, and who might not be there in three months time. The sales floor often has a high overturn of staff, most have no loyalty to the company.
So the above is a small sample of some boring factors that may affect the quality of a magazine we see in the store.