Partitioning question - after sliding in fragmentation increased!

Hi, this is something that confuses me, and i don't have real explanation for it:

We load data into partitioned table by sliding partition in: first we truncate staging table, load it with data, create indexes on it (according to the partitioned table), slide this staging table into partitioned table as one of partitions, and somehow, dm_db_index_physical_stats shows that slided partition is fragmented around 50%. Every time we do it. Until i defragement that partition.

That doesn't make sense to me, because we rebuild indexes on the staging table just before sliding in. Why would sliding in change fragmentation of partition?

Any explanation anybody?

Thanks, Pixie