The dishes are microwave safe and it only happens when thawing frozen vegetables.
Are the vegetables in anything else? A pouch, a container?
Try putting just water on the microwave safe dish and see if the dish truly is that.
In general, sparks in the microwave are caused by electrical energy jumping airgaps between large voltage differentials in a conductor. The simplest such arrangement is a metal ring with a gap of ~1/8[sup]th[/sup]inch cut into it. The metal ring acts as an effective antenna, and impressive flares can be generated with this arrangement**. My guess is that some parts of the mass of frozen veggies is acting as a conductive loop antenna, and sparks are jumping airgaps at various points, until thawing stops the conduction.
**Don’t try this unless you know what you’re doing.
They are truly microwave safe.
Thanks, Q.E.D. 
Certain fruits will spark in a microwave. If you cut a grape in half at its equator, and microwave it, you will see sparks dance at the center. So maybe some vegetables will as well.
I’ve seen this with frozen corn.
It’s quite odd - and yes the dish was microwave safe.
Anything that produces a charge imbalance can produce sparks. When dealing with microwave ovens, metal is the most common culprit, but other things can cause it as well. Electrical charge tends to a) accumulate on the surface of an object and b) be greater on pointy protrusions; both effects result from electrons repelling each other. I suspect that your frozen vegetables provide a large number of nicely pointy discharge points–either the veggies themselves or the ice crystals on them. The microwaves cause a charge to build up at these points until they start to arc.
It might help if you were to spritz the veggies with a little water before defrosting. Ice is significantly less conductive than water (and doesn’t absorb microwaves as well), so a film of liquid water may conduct the charge away before it builds up enough to make the jump.