I think Hortic has got it absolutely right!
Lots of our tom trusses are similar - I saw some an hour ago - and I'll put it down to a very hot June, when most respectable toms are getting their wits about them, then realising that it's really hot enough for them to do a 'summer job', so leave out some of the 'titches' to fend for themselves for a while! They can maybe catch up later...
I've never grown San Marzano before, but am amazed at how prolific they are, and how quickly they ripen - often after picking, they reddened after just a few hours! What a fabulous tom they are!
You guys here recommended San Marzano, so we're very grateful!