I suggest the term 'project dependencies' or 'dependencies' for those tasks that 'cause you to get side-tracked'. Also 'critical task' is a term you can use.
The underlying activities in a project, on which other, and often more important activities or 'deliverables' - things that must be delivered or completed in the project - depend - are known as 'dependencies' or 'project dependencies'.
This is a project management term - a term from the world of 'project management' a profession in which project managers endeavour to keep projects on track and to generate successful, timely outcomes in accordance with 'the project plan' or the stated project goals.
As projects are commonly conducted in business, the term 'dependency' or 'project dependency' or 'critical task' will often be used in business meetings about projects, including software development projects, and other projects.
Another name for a key 'project dependency' is a 'critical task' - a task on which many others, - depend. And on which, the success of the project also depends - which is why these are deemed 'critical'. Meaning 'critically important' - to the success of a project.
A 'critical task' may be seemingly minor - but can have huge knock-on implications, if not achieved. For example if a logo design is incomplete, an entire marketing campaign might falter.
So to come back to your question - you can talk about 'underlying project dependencies' or 'unexpected project dependencies' or 'suddenly uncovered project dependencies' or 'suddenly occurring project dependencies' to cover the 'tip of the iceberg' feeling that is causing your delays.
Or even 'emergent' or 'emergency' project dependencies, or 'newly emergent critical tasks'.
Here is a link explaining dependencies:
Successful project management often involves uncovering the 'underlying project dependencies' which may be defined as 'that, without which, the project cannot continue or progress' - as early as possible in the project - so that they may be addressed - so that they do not, in fact, get in the way of progress - on indeed, stop the project altogether.
The project manager's task is often, in fact, to weasel out these hidden rascals - and find ways to deal with them.
Most project management software has features to 'identify dependencies' and allow them to be seen - which is designed to allow the effect of them slipping on other areas - to be seen - and then, hopefully - resolved.
The dependencies, when they slip, can have dramatic knock-on effects on other activities, where a delay in one area creates a kind of negative 'domino effect' on the whole project.
Various project management methodologies exist to assist with the whole problem you describe, including 'Agile' and 'Scrum' - here is a link to an article summarising some of the available methodologies that may be helpful:
Lastly, another way of describing the whole problem you describe is 'failure to apply project management principles' or 'unsuccessful project management' or 'a lack of project management'.
A project manager in a tricky situation such as yours, might say to management 'my dependencies are slipping' or 'a number of previously unidentified critical tasks have come up which are impacting the project'.