What's that saying?
"Cui bono?"
Let's parse this out.
In a 3-way race, Sinema would have lost to either Lake or Gallego.
She'd be out any campaign money she raises. Perhaps even in debt.
The DCommunists could lose the seat (if Lake ekes out a plurality).
Neither Sinema nor the DComs benefit.
In a 2-way race (without Sinema), there's a very good chance Gallego will beat Lake.
The DCommunists win the seat, finalizing Arizona's status as a blue state.
Sinema is spared an embarrassing loss (perhaps financial debts, as well).
Both the DComs and Sinema (slightly) benefit.
While Sinema may have considered withdrawing on her own, I sense (one of those things you don't know but would certainly seem plausible) that the DCommunists have made her "an offa she can't refuse"... to step away and give them a more certain shot at the seat.
Lake lost once (to outright fraud, when the Pubbies were still in nominal control of Arizona).
Do you really, really believe with the DComs now in control there, that they'll let her win a second state-wide race?
(I'd enjoy being proven wrong, but I call 'em as I see 'em, and that's how I see this one going...)