Hello Gareth Monkton, welcome to Psych Central.

I can't talk for everyone that may suddenly decide to do an irrational act but people with bipolar can have symptoms that lead them into ....
During a manic phase, symptoms include:

• heightened sense of self-importance
• exaggerated positive outlook
• significantly decreased need for sleep
• poor appetite and weight loss
• racing speech, flight of ideas, impulsiveness
• ideas that move quickly from one subject to the next
• poor concentration, easy distractibility
• increased activity level
• excessive involvement in pleasurable activities
• poor financial choices, rash spending sprees
• excessive irritability, aggressive behavior

These are not purposeful acts that have been thought through properly, that's the difference between someone that knowingly steals for example. Someone with bipolar might realise the next day that they spent all their money on the internet the previous night but they hadn't purposely set out to do that. There is an element of delusion and often psychosis with bipolar and they often feel great horror and remorse at their actions. Here is a link to symptoms of bipolar... Symptoms of Bipolar Disorder (Manic Depression) | Psych Central