I expect if you were to tip the tray out you would find a mat of roots at the base.I think you may have started them too soon, or should have planted them out 2 weeks ago if conditions were fit.
Basically they have done well, with the provided conditions but I feel, have outgrown their surroundings, depleted the compost and are now responding to the stress by a last ditch attempt to seed before its too late!
Spinach is especially prone to this, some varieties will bolt almost while you watch. Suggest you harvest the good baby leaves,to take some crop for your efforts, and maybe sow more outdoors now.
