That dropping of oldest leaves, in general older than three years, is normal for rhododendrons. You can't make them stay in place :)

Some rhododendrons are just normally leggy, others will become so depending on their growing conditions, but you can play a hand in their ultimate shape especially if your plant has only been in the ground like you'd mentioned for a year.

In order to encourage branches to form, pinch terminal leaf buds (not the round, fat, blunt-tipped flower buds) as growth begins and they are starting to elongate in Spring. This will force shoots to develop from dormant eyes in the lower leaf axils.

A plant hormone is produced by leaf buds which inhibits development of dormant buds along the stem; flower buds do not produce this hormone. Continue this pinching procedure for each growth cycle for two or three years - by then you should have a well fill-out shrub what will produce many more flowers buds than one left to its own devices.

After this initial training, your rhododendrons basic structure is established. You may still want to pinch leaf buds which would grow to unbalance the shape, however, since healthy rhododendrons produce more terminal flower buds and less terminal leaf buds as they grow older, your training duties decrease :)