It's kind of odd that they did not accept volunteers.
I've survived (?) RIF's coming about every 6 months over the past 5 years. My company was split - (rif's before and after), sold (rif's before and after) then my division was sold last year (big rif's after that one.). They never asked for volunteers... though I let bosses know I was fine if they wanted to let me go and save someone else's job.
19 years at this place, on my 7th payroll corporate title, and never once have seen them ask for volunteers.
Rumor of another RIF coming next month or the month after... A severance would definitely kick me out of OMY.