It's just an accounting glitch

Sanders campaign started April 30. In this Politico story, the campaign stated they had raised $13.7 million from April 30th to June 30.

"76 percent of the Vermont senator’s donations were $200 or less, amounting to just under $10.5 million of the $13.7 his campaign raised through the end of June.
The $15.2 million comes from a transfer of $1.5 million from his Senate campaign committee.

http://www.politico.com/story/2015/07/bernie-sanders-campaign-finance-fec-15-million-2016-120164

There were several stories and campaign announcements regarding the amount that matches the report:
http://www.cnn.com/2015/07/02/politics/bernie-sanders-fundraising/

Campaign Finance Institute has two charts that support the reported numbers:
Here: http://www.cfinst.org/pdf/federal/president/2016/Table1_Rec-Cash.pdf

And here:
http://www.cfinst.org/pdf/federal/president/2016/Table2_Individual%20Donors.pdf


At the end of the day, nothing "mysterious" but a staffer making a mistake in the paperwork.