The market of skilled hardware designers is running low: training costs are high, complexity has skyrocketted. By doing this they can increase attention to a field that is otherwise dominated by big corporates (already somewhat the case). The only way to have a sane and healthy chip market is to make 1) the entry barrier low and 2) stimulate innovation. This does both of that.

They want to spur competition among silicon suppliers.

The industry has become dangerously too consolidated, with like of Avago/Broadcomm trying to buy themselves a monopoly.

The big semi look at big dotcoms like Google as cows to milk, obviously they don't like it.

Google wants to create an open, innovative ecosystem for silicon so it will be easier for them to build accelerators for their workloads to meet the growing demand for compute. TPU is only one example of the kind of accelerators they want to build. Tim directly addresses this in the talk: https://youtu.be/EczW2IWdnOM?t=407.

I know Google absolutely loathes having Intel silicon in their datacenters, the management engine and other blobs can't be audited. It's conceivable they want to help bring open chips to market to try and remedy this problem.

I wrote something similar above, but maybe a bit like Microsoft acquiring LinkedIn? To get a list of chip designers that could possibly work at Google? Since the designs are open source, they can also evaluate their skill level. And lastly, the contributors are probably less likely to already work at IC companies that have NDAs, etc.

Possibly a new source of intellectual property? I would be interested to read the terms and conditions.