IMB Bank's share price usually follows a steady, uneventful course, much like the financial performance of the mutual bank itself. But since the beginning of November it has suffered a steep fall that IMB is at a loss to explain.The share price has traded between A$5.20 and $5.60 throughout most of the year and hit a peak of $5.71 on September 23.But since the start of November the price has been up to as high as $5 and, last week, as low as $4.20 - a range of 16 per cent. The last sale on Friday was at $4.50 a share.The fall from the peak to last week's low of $4.20 was 26.4 per cent.IMB shares are traded on a market operated by IMB, which is the holder of an Australian Market Licence. The market's operations are supervised by the Australian Securities and Investments Commission.Last Wednesday the mutual bank issued a statement saying: "There has been increased trading activity and various price movements on the IMB market following the release of recent market announcements. There is no additional price sensitive information of which the board is aware beyond that which has already been released."In July the bank completed its merger with Sutherland Credit Union. In August it reported underlying earnings that were in line with the previous year.On October 20 it launched an off-market share buyback - the fourth in a series of buybacks intended to cancel its ordinary shares.The mutual bank said it would buy up to $18 million of its ordinary shares through a voluntary tender process. The buyback price will be determined at the end of the tender process. The price will be at a discount of up to 14 per cent of the volume weighted average price of IMB shares over the ten days on which they have traded prior to October 20 ($5.46), adjusted for the movement in the ASX 200 index from October 20 to December 7.It would be a surprise if the buyback was causing the share price to fall. The buyback will have the effect of increasing earnings per share and should, if anything, cause the share price to rise.