why are all my beefsteak toms all going brown

Hi Victor welcome to GoY, sorry to have to tell you but you've got blight and need to destroy, buy burning or binning, the complete tomato plants. Don't put them in the compost as the virus can remain. If you planted them in containers bin the soil too, if in the ground don't grow tomatoes or potatoes in that area for a few years. Blight has been extremely common this year and we've had lots of folk asking what the problem is.

Sounds like potatoe blight is your problem.
Tomatoes are from the same family as spuds and the wet/warm weather is playing into the hands of blight everywhere.
Remove/ destroy rotten growth and lessen watering regime. Plant tomatoes elsewjere if poss next year.

It's probably blight at mentioned above, but if the brown starts at the bottom of hte tom and gets wider in a circle, then it could alternatively be blossom end rot, which is caused by irregular watering/feeding.