This is puzzling at best, but it could mean that Brexit was merely a catalyst for a more general re-evaluation by markets of the amount of short-term government bonds available to investors, possibly related to expectations of central bank purchases or in anticipation of perceived political risks, such as the US election, which was just a few months away.
Events in recent months also have proved surprisingly persistent. Market intelligence generally pointed to the re-emergence of perceived political risks in the euro area as a prime reason for the renewed marked drop in short-term yields of German bonds in February. You can see on slide 5 that this drop indeed coincided with a widening of sovereign credit spreads.