"Seething breeding" is not an idiom, although you can find the two words together on occasion. I think the problem you may be having is in parsing the words next to seething breeding. In the two examples you cited, the noun is a breeding ground, and then that noun is in turn seething. To bracket it simply, it is:
[seething [breeding [ground]]]
A breeding ground is:
A place or set of circumstances that encourages the development of certain ideas or conditions
So a seething breeding ground is a breeding ground which is in a state of excitement or agitation.
Now, I am not in the medical field so I am fairly uneducated in what a glioma is. Having looked it up, I am aware that it is a type of tumor. I believe that the same brackets can apply to help parse the phrase:
[seething [breeding [glioma]]]
That is, the glioma itself breeds (probably by spreading through the body, creating other tumors). So it is a breeding glioma. This "breeding glioma" is also in a state of agitation, or it is seething--thus, it is a seething breeding glioma.