Broken main stems - why?

[Tomaddict]

I have found two tomato plants (and one pepper) in my garden with broken main stems this year. This was noticed approx. 3 weeks after transplanting large, sturdy, hardened seedlings to the garden. The breakage is occuring 2-3 inches above the soil, and as you can see in the photos also a bit above the level of the straw. The tomato plants were inside cages, set in place about 2 weeks after transplant, and the plants seemed to be strong and healthy at that time. With both tomato plants I noticed the stems were hollow, as you can see in the close-up photos of the Mayo's Delight. The photos of the Lucky Cross were taken about a week after the injury, and the hollowness of the broken off stem is mostly "healed" shut. I don't think hollow stems are normal, don't they usually have a pithy interior?

I have not noticed any tell-tale signs of visitors, like droppings (and it's a month too early for hornworms to emerge here) or animal tracks (it was fairly muddy when the Lucky Cross went down, and the garden is surrounded by multiple layers of fencing, not that it keeps out little critters like chipmunks, but the cat keeps them under control for the most part.)

Well it's not a crisis by any means since I have boatloads of other tomato plants to keep me amused (including others of these varieties, thankfully - oh no, not my Lucky Cross ), but I'm wondering if any of you may have seen this phenomenon or have any ideas about what might cause it.

BTW I was able to save the top of the Lucky Cross and am rerooting it, and it seems like it'll make a go of it. The Mayo's Delight broken top was pretty much a goner when I found it so I didn't try to salvage it.

If you have any ideas about what might be causing this, please let me know!