The data that you see is not a representation of what everyone else sees. The data that you see is solely based on the number of transactions Blockchain.info has on their mempool, everyone can have different numbers due to poor propagation of transactions or differing rules.
The drop in the mempool transactions is likely triggered by a restart of the nodes that they have. The mempool might have been emptied by Blockchain.info and the increase in the number of transactions is due to nodes sending more transaction data to Blockchain.info after that.
Thank you for these explaination.
It is most clear in my mind now, specialy the restart of the blockchain.info node :-).