Maybe it can happen. I have traded eurex bunds during 5years. I was sitting next to the eurex guys (the bank was an eurex member) and I have never heard of such things. Moreover, if someone comes to me and asks a bid in 100mio eur 10yr bunds, I will make him a straight bid for his size and will take care of the position myself (hedging with bund futures... of course with slippage if I have been hit in 100mio). But it's true that these spikes in volume are on the charts, so your explanation could be a possibility.