This is a compatibility issue with DW CS6 and Yosemite. it is caused by a memory leak in DW with large GUI's. We are looking into finding the exact cause of the problem so that we can fix it.
For now, some things that may help:
restart the OS, then delete the mac Cache File before starting DW so it is the only application open
Close all panels like the server behaviors, Files panel and properties inspector before starting one of the UIs
Go to the WebAssist -> Preferences and disable the preview.
These steps won't necessarily cure the problem, but may slow the frequency.