This has happened to me a great number of times, but seemingly totally randomly.
1) I click the x to close a tab, or I forget to drag-and-drop a new search tab and just click the search icon instead.
2) Instead of the expected result, I find myself staring at an, often great, number of unwanted search tabs opening all over the screen, usually in new panels. Like this, for example:

Or like this:

Except this last one shows the only time when it didn't happen after one of the actions in 1), but instead -- if I remember correctly -- after clicking either Basic or Bible in the one search tab I already had opened.
I think this should be the log from that occasion: 7115.Logos4.log110426.zip (The screen shot was taken at 01.40.59)