It could be lacking nutrients. Pelargonium's in pots like a regular feed or it may have a virus

I have often wondered about this and my theory is that it is in the nature of the plant to produce leaf growth until it wants to reproduce,i.e flower.

Flowering seems to take place when the plant becomes a little distressed.

Stress can be manipulated in various ways,e.g. Allowing the plant to become pot bound,letting the plant nearly dry out before watering and pruning or a permutation of these

Once this happen the lower leaves which could well be shaded by the upper leaves become superfluous, die and fall off.

As I said this is only my theory, perhaps someone else has a more knowledgeable answer,so I will look on with interest for such a comment!

Pelargonium's do require lots of water.

Watering until "just moist" may not be enough amd may be causing stress unless done very regularly .

I stand mine in water until the compost stops sucking out water ,then let the compost get nearly dry again before watering .

That looks as if its suffered from drought to me - you can water from above, I always do by applying the water to the compost, not the plant. What's important is drainage in the pot - as long as the water can freely drain from the bottom, and isn't into an outer tray or pot which doesn't drain, then give the compost a good soak.

I do what Anchorman does if the pots are small enough to go in rubber bucket under the butt tap. They love it. At this time of the year geraniums go off the brew a bit after all their hard work flowering. The ones in big pots don't yellow their leaves as much as those in the smaller pots.....suggesting stress as a cause.

Thank you all for your input - I will take it all on board. In fact today I have given it a good soak by standing it in a basin of water.
Was afraid of over watering so was probably being over cautious - especially with the amount of rain we have been having.
I've picked of all the crispy brown leaves and I can see that there is tiny green shoots - so fingers crossed I can get it going again.