You've just described a lot of people just before buying a house I suspect. Loads of people act in the same way and will buy a house having inherited or having been given gifts from the bank of mum and dad. Or sold something. Or possibly even won gambling. Why do they care where the deposit came? £25000 from Mum and Dad surprisingly works as well as £25000 from a Casino.
AML (anti money laundering) regulations. Believe me its insane now. The lender I work for goes a little over the top in my opinion, the solicitors are also supposed to do their own AML checks but we question a lot of stuff - I don't make the rules.
Get particularly messy with deposits coming from overseas.
But you know, people all blamed the banks for 2008, and now people complain its more difficult to get a mortgage, which do you want?