It is probably just old leaves dropping off further back down the stems. This is quite normal around this time of year for many evergreens. As long as it is making good growth too it should be fine. The leaves normally last about 18 months to possibly 2 years before dropping so there should be this years new growth foliage, then just below hopefully most of last years with maybe the bottom few starting to drop, and below that leaves dropping.

You can leave the shrub to grow unpruned but it's a common practice for Photinia's especially the ones with red or bronze new growth foliage like this variety to be pruned back to encourage bushiness and lots of new shoots. This can be done either before the new growth starts in mid spring or around now but not much later than this. Feed with a good broad spectrum slow release fertiliser such a Q4 twice a year.

Mine drooped a lot of its new red leaves at the top this year but I gave it some fish and blood and watered it I think mine was due to the very dry spring.