I agree with Sandi, body trauma can trigger it.
My husband of 33 years died suddenly just over 7 years ago when we were both 54.
I reacted by developing anorexia and lost an awful lot of weight. With tremendous support from friends, Occy health and my colleagues I was back on 12.5 hours shifts doing the job I loved after 4 months, Senior Staff Nurse on a Level 2 high dependency unit. A very stressful working environment however.
6 months later my eldest son (then 30 with a 4 month old son) had life saving open heart surgery after a congenital defect was discovered. I went completely over the edge. Occy health were great, diagnosed PTSD and got me in for treatment which lasted over 2 years.
May 2014 I was happily back at work, although still trying to gain a few more pounds in weight, when I developed the most horrendous viral sinusitis. I was ill for almost a fortnight.
6 weeks later after a fairly insignificant gardening accident left me with purple / blue from thigh to toes on one leg with several blood blisters the size of eggs I was admitted to Haem ward with a count of 12.
I know my count was ok a few months before that as a previous incident, not my fault this time, resulted in the bruising I expected.
After every test under the sun came back negative the general consensus of opinion amongst medics (and I can't argue against it) was that after 3 years continual stress, of one kind or another coupled with chronic under nourishment my immune system could take no more and forgot to switch off the antibodies once the sinus infection was resolved. My platelets became their prey. Spleen thinks these antibodies aren't needed anymore so gets rid of them along with the healthy platelets attached to them.
An absolutely fascinating process to watch on an Indium scan by the way.