First of all, can anyone explain what causes such a sudden sharp rise in number of transactions? This just happened now.

That is a bit of a misleading graph, it's not tx per second, it is in vb/s.
So assuming I'm the only user if I send a consolidation one like :
https://mempool.space/tx/c5d2b9d5ae8941f05fd9be2a040564ab0c81f9af3cacde4bf3862e5c4a5299e7
that is ‎43 000 vb/s
if I send 10 of this
https://mempool.space/tx/9021a8d8061c5201aac5441e9bb171c572d2d9a62fa94e7a9a46948b3f809741
‎it's 1920 vb/s

Basically any service consolidating will make a spike in that graph.