It looks like something caused your powder to absorb excess moisture. Either that or you still have excess powder on the cards.
Fanning powder used to be a big deal when decks were more expensive relative to people's incomes - these days, few people use it because cards are fairly cheap and the coatings most well-made decks come with (assuming you aren't buying dollar-store, bargain-basement decks) are more than adequate to giving the cards good slip over the life of the deck. I know bog-standard Bicycles are pretty cheap in the States but a bit harder and more costly to come by in the UK, so I can understand your desire to get the most life out of them that you can.
You also might get better results if you apply the powder to the cards individually. The dump-em-in-a-bag method is pretty haphazard as far as insuring good, even coverage on all the cards. Consider using a clean, unused brush, something soft like what a woman might use for applying blush or concealer or what a forensic team would use to dust for fingerprints, or even a big artist's brush that's adequately soft-bristled. Apply it VERY lightly - a little goes a long way, too much is no good. If you can't find any of that, try a small amount of powder and two hand towels - place a small pile of powder on one towel, move the card over and in contact with the towel to get the powder on, use the clean towel to get the excess powder off.
When handling any cards, powdered or not, make sure your hands are COMPLETELY CLEAN. Use a greaseless moisturizer on your hands, making sure it's completely absorbed into the skin before handling the cards. And accept that your spectators will NEVER treat your cards as well as you do - to them, they're playing cards, a cheap, disposable commodity, nothing to get excited about, whereas to a magician, they're the tools of the trade, especially if the magician also happens to be a collector! I've almost completely stopped performing randomly for people with costly or rare decks, using only the cheap, easily-obtained stuff - I know they're going to get mishandled so it's easier on my budget that way.