It's an exothermic reaction, also known as a 'heat spike'. It's usually more noticeable with gels when curing in the lamp, but the same chemical reaction takes place with L&P but at a slower rate so they client doesn't feel it. Although if you were to touch the L&P as it cures you would feel it warm slightly whilst hardening.
The reason your clients are feeling it more now is because of the unusually hot weather we are experiencing, it always happens in the summer, for a number of reasons....clients are natually already hot so any increase in temperature is more noticeable.....the amibient temperature will affect your products, hence crystalising in the winter and heat spike and monomer evaporation in the dappen dish in the summer. The L&P will set up faster so clients are more likely to feel the heat spike. You will also probably, unknowingly, working dryer, as the monomer evaporates from your brush and whilst the bead sets up, working dry can also cause the heat spike.
So all in all, it's because it's hot