Explanation/Reference:
A queue with strict-high priority is assured unlimited transmission bandwidth but is not actually assigned a large delay buffer. Not configuring a transmit-rate or an explicit buffer-size on a strict-high priority queue only ensures that the queue gets assigned a default minimum delay buffer, making it possible, under bursty conditions, to see tail-drops on strict-high priority queues. Assigning a small transmit-rate or an explicit temporal or percentage buffer-size to the queue ensures that the queue has a large enough buffer to hold bursts and protect against tail-drops.
References: http://www.juniper.net/documentation/en_US/junos15.1/topics/usage-guidelines/cos- configuring-schedulers-for-priority-scheduling.html