The S&P/ASX 200 Index (ASX: XJO) was on form in May and recorded a 0.75% gain over the month.
Unfortunately, not all shares climbed with the market. Here's why these were the worst-performing ASX 200 shares in May:

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Tuas Ltd (ASX: TUA)
The Tuas share price was the worst-performer on the ASX 200 index by some distance with a decline of 65%. Investors were selling the Singapore-based telco's shares after it terminated its proposed S$1.4 billion acquisition of M1 Limited. Tuas made the move after authorities learned that its Simba business may have been using radio frequency bands it was not authorised to use. To fund the acquisition, Tuas undertook a A$416 million capital raising at $5.51 per new share. It is unclear what the company will now do with these funds.
Tabcorp Holdings Ltd (ASX: TAH)
The Tabcorp share price was a poor performer and lost 32% of its value in May. The catalyst for this was news that the gambling company has become the subject of an AUSTRAC enforcement investigation. Tabcorp advised that this relates to anti-money laundering and counter-terrorism financing (AML/CTF) compliance. AUSTRAC has stated that the investigation is at an early stage and its approach will be determined once sufficient evidence has been collected and assessed. In response to the news, Tabcorp's CEO, Gillon McLachlan, said: "I am committed to leading a compliant and safe company that understands its risk obligations. Uplifting our risk capability has been an ongoing part of the Company's transformation and we will work constructively with AUSTRAC through this process."
IDP Education Ltd (ASX: IEL)
The IDP Education share price was out of form and sank 32% over the month. This was despite there being no news out of the language testing and student placement company. However, late in the month, the team at Macquarie downgraded IDP Education's shares to an underperform rating (from neutral) with a reduced price target of $2.35 (from $5.45).
Brambles Ltd (ASX: BXB)
The Brambles share price had a tough time in May and dropped 27%. This was driven by a guidance downgrade from the supply chain solutions company. Brambles revealed that it now expects sales revenue growth of 2% to 3% (from 3% to 4%) and underlying profit growth of 3% to 5% (from 8% to 11%). The company's CEO, Graham Chipchase, said: "Our immediate priority is to meet our customers' needs and to restore stability and service in the affected parts of our US network. Our response and ongoing investments in quality reinforce that meeting our customers' needs is non-negotiable. We will not compromise on the investment required to meet the quality, network resilience and service outcomes our customers expect."