one thing to consider is the per-storage-account scalability target of a storage account. With georeplication enabled, you have 10Gbps egress and 20K transactions/sec, which you could be bumping into. Figure with 150 instances, you could potentially be pulling 150 x 100Mbps, or 15Gbps as all of your instances are starting up.
Not sure about the "mounted VHD" part of your question. With Azure's drive-mounting, only one virtual machine instance can mount to a drive at any given time. For this type of file-copy operation, typically you'd grab a file directly from a storage blob, rather than a file stored in a vhd (which, in turn, is stored in a page blob).
EDIT: Just wanted to mention that an individual blob is limited to 60MB/sec (also mentioned in the blog post I referenced). This could also be related to your throttling.