Have you made sure that all of the sims that you consider "played" sims are favourited in Manage Households by clicking the heart button? Usually the game will leave sims in this tab alone, while every sim in "Other Households" is considered an unplayed NPC and open to having their skills changed, relationships erased and jobs reassigned.
Also, the game has a ratio set for unemployed sims vs employed sims (which differs by age group). So for adult sims it's something like 80% employed and 20% unemployed. I found that when I had a lot of sims with jobs I had set, one day I found a whole bunch were randomly switching jobs because the game had made them unemployed.
Both of those things has been in the game since way before Eco Lifestyle. One thing I find immersion breaking is the new tendancy in recent packs to re-use sims for pseudo-jobs (it doesn't change their career, but puts them in a role of an NPC temporarily). Like for Eco Lifestyle, that's the NAP Inspector – there's no actual NPC career for that, it just seems to pick a random unemployed sim. In Get Famous, it does the same thing for bouncers guarding VIP doors. In Island Living, it does that a bunch for island events (but those are usually OK story-wise because it uses island sims "volunteering" to do things like run stalls for the community or be a lifeguard).
