Here are few general sources of wealth transfer and redistribution:
1) Military power/colonization (War);
2) Economic colonization;
3) Political colonization;
4) Legislation;
5) Taxes;
6) Labor; Hard work;
7) Markets; Monopoly/oligopoly;
9) Speculations;
10) Fraud/Crime/etc.;
11) Natural disasters;
12) Culture, traditions;
13) Inheritance.
14) Social programs / Gov.
15) Contracts, deals, trade, exchange, etc.;
16) Charity;
17) Random events.

But in general main source is relative weakness of those from whom wealth transferred, thus motivation and relative strength of those to whom wealth is transferred.

Another way to look at this is - wealth transfer sources are formations which does have wealth - so to transfer something from someone to someone we had to have this something at initial source and also source to where should exist.