Just a couple of suggestions but this may be what's going on.
1. With most native fruit and veg there are early, mid-season and late crop varieties, so perhaps Tesco is currently stocking a variety of greens which some rabbits and guinea pigs find less palatable. If this is the case, the variety will probably change again in a few weeks.
Or
2. The current stock is coming from farms in a different part of the UK with different soil characteristics and trace elements, so the greens may be the same variety but taste different to rabbits and guinea pigs.
And of course it could be a bit of both.
In case anyone's interested, Tesco seem to have significant reductions on many of their pots of fresh herbs at present. I've just picked up a mint for 99p and the thyme is a bit over a quid.