I would say uneven weather and uneven watering

If its hot they take up lots of water but sometimes the skins have begun to ripen, as the flesh inside expands the skin splits.
They really like to constantly have enough water.....and for the temperature always to be the same.....both are well nigh impossible domestically!. we are at the mercy of the weather and our busy lives.

I take them off as soon as possible,. Wash them really well and usually use them for cooking

Thanks for that Pamg, we have been experiencing rather uneven weather haven't we? The uneven watering I will blame my husband for!