1. At peak popularity, Borland products where easily available. Borland decided to turn to enterprise and raised the price considerably, so individuals and small companies started looking elsewhere. By the time they realised the mistake it was too late. In my opinion this was the biggest mistake.
2. Internet and Linux came, and with them Perl, PHP, Python and others. Borland missed the boat, and again by the time they realised that, it was too late.
3. Sun came with Java and Microsoft with C#, both seen as the future of enterprise, and available for free or at very low cost. Java was extremely popular at education sector, pushing out Pascal and other competitors. Both made Object Pascal obsolete.
So bad decisions and being late to the party. Also it was hard to compete with Microsoft in the long term.
As an unrelated sidenote, at the time when world was turning towards agile, they were building and marketing software for managing waterfall project management. That just shows how disconnected from reality of their customers they were.