Z, in our own state (I believe you are in Washington also; correct me if wrong), that really didn’t happen in general when Medicaid rolls increased in 2008-2009. However, a small number of individuals were notorious at racking up excessive numbers of ER visits — some practically every day. The state implemented a plan to limit non-emergency ER visits to 3/year. But this was thrown out by a court I believe and the limit policy was dropped subsequently. The state now tries to vector those patients to urgent care clinics and/or mental health, whatever is appropriate.
My understanding of Medicaid here, before ObamaCare, was that only children, pregnant women, SSI Disability, and people waiting for a Disability decision were eligible.
In other words, no able bodied adults qualified for Washington state Medicaid, regardless of income.
Under ObamaCare, if I'm correct, all adults and families who earn less than 130% of the poverty rate (so, below $15,000 for a single adult) qualify for Medicaid.
For several years before 2014, Oregon's Medicaid was income based, too.
It was those people, the “new” people in Oregon on Medicaid, who dramatically increased their ER visits.