Essentially the problem is that the writing style and vocabulary in Timothy 1, 2 and Titus (the pastorals) are distinct from other writings attributed to Paul. So much so that it is unlikely to have been written by the same person.

Many of the words and phrases in the pastorals aren't used by Paul anywhere else, and are more familiar to 2nd century Hellenic Christians than Paul. More damning is that the pastorals sometimes use the same words but with different meanings from Paul.

The author writes of things that would have been alien to Paul, such as elders, deacons and bishops; describing a church structure that didn't exist in Paul's lifetime.

While "a textual power struggle was under way to redefine Paul’s intentions with respect to women" is a matter of interpretation, this unknown author appears to hold distinctly opposing attitudes to Paul.

Timothy's author is in stark contrast to a comparatively inclusive Paul, who emphasises the importance of faith in Jesus over works in finding salvation. The Pastorals lay down the law, barring women from the church, condemning heresies, emphasising the importance of church hierarchy, etc, all of which is detached from Paul's efforts to spread a fledgling faith.

The book "Forged" by Bart Ehrman explains in more detail, although I think the authenticity of the pastorals have been in doubt far longer.

It wasn't uncommon in the early church for people to pass off their own thoughts as coming from a more eminent author (including St Paul and St Peter), simply because it was very hard to detect forgeries. In the case of 1 & 2 Timothy there is strong evidence they were written far later than Paul's epistles for a number of reasons:

• the description of the church (notably bishops - which mainly developed after the death of the apostles) is of a 2nd century church - unlike Paul's earlier letters.
• the references to Paul's travels can't be reconciled with Acts.
• the 'tone' of the letters is wrong - Timothy travelled extensively with Paul (see Acts) yet the letter is impersonal & some advice in it he should already know.
• the language is different to and more strongly reflects common 2nd century usage than Paul's earlier epistles.

I am not sure if it is within the rules of this subreddit (and please remove this post if it is not) to simply point out an interesting observation here. The Nestle-Aland work on the New Testament states that there is the lowest percentage of variant free verses version to version included in the earliest seven Greek translations of the New Testament (just over 81% do not vary).

Source: K. Aland and B. Aland, The Text of the New Testament: An Introduction to the Critical Editions & to the Theory & Practice of Modern Textual Criticism