As far as I can see there are two senses in which it's 'rational' for an investor to buy during a bubble.
- The investor has erroneously overvalued the value of the stock/commodity.
- The investor is aware that it's a bubble and is overvalued, but believes he can make a profit by selling before the bubble bursts.
There are also 'irrational' explanations, such as the investor being caught up in the hype, or fearing missing out.
The question is, is investor behaviour during bubbles well studied, and are there models explaining it?