This appears to only occur if until the size of the high priority transactions reach a limit set in the configuration:

https://github.com/bitcoin/bitcoin/blob/master/src/miner.cpp#L120-L123
https://github.com/bitcoin/bitcoin/blob/master/src/miner.cpp#L222


If -blockprioritysize is not set (or is set less than or equal to 0), then the transactions will all be sorted by fee and priority will be ignored for fee paying transactions.