Define the problem and collect data:

First, I would confirm that there is indeed a 25% increase in bounce rate and that it's not an error in data collection or reporting. I would also want to see when this change happened - was it a sudden spike or a gradual increase over time? This will provide the initial context of the problem.

Break down the problem:

I would then try to dissect the problem further by segmenting the data. Are specific pages experiencing higher bounce rates than others? Is this problem more pronounced with a specific type of user, or is it across the board? Is the bounce rate higher on specific devices or browsers, or from specific regions? Understanding this can help pinpoint if the problem lies in a specific feature or if it's a broader issue.

Identify internal factors:

Internal factors could be any changes that we have control over, such as changes to the website design, user interface, loading speed, broken links, or implementation of new features. I would look into any recent changes or updates that have been made, especially those correlating with the increase in bounce rate.

Identify external factors:

This would involve factors that are not under our direct control, like increased competition, changes in customer preferences, market trends, seasonality, or any significant events in the news that could affect online shopping behavior.

Examine relational KPIs:

Look at other metrics that could be related to the bounce rate, such as time spent on the website, page views per visit, or conversion rates. This can provide clues about user behavior and possible reasons for the increased bounce rate.

Involve the relevant teams:

Engage with the technical team to check for any bugs or performance issues, the design team for any recent UI/UX changes, the marketing team for changes in advertising or targeting strategies, and the customer support team for any increase in complaints or issues reported by users.

Develop hypotheses and test:

Based on the findings, develop potential hypotheses for the increased bounce rate. For example, if we recently implemented a new feature and the bounce rate increased since then, it might be a cause. Or if a competitor launched a big sale during the same period, it could have influenced our bounce rate.

Plan and act:

Once the root cause is identified, develop a plan to address it. This might involve fixing bugs, improving site speed, revising design elements, or adjusting marketing strategies. Also, ensure to set up a system to monitor the effects of the changes made to validate if the bounce rate improves.