Most of the UBS/MUFG shares aren't actually owned by UBS or...

Most of the UBS/MUFG shares aren't actually owned by UBS or MUFG. Mitsubishi (MUFG) is reporting their holdings because of their ownership interest in Morgan Stanley. Most of these UBS and Morgan Stanley shares seem to be held via their prime brokerage divisions. You can see some of the counterparties on the last few pages of the MUFG substantial shareholder release on 24 February (they're event-driven/ merger arbitrage funds).

It looks like much of the abnormal trading volume is due to hedge funds (or other entities) that have taken a long derivative position in WSA (e.g. via an equity swap), with the prime brokers hedging their exposure by buying the physical WSA shares. They're betting the $3.36 offer price gets improved on.