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That is called a ligature, turn off that option in OpenType functions if you need to in InDesign.
That's a ligature. You can turn off ligatures from the character panel (or as part of the style definition). In this case it would be under OpenType Features > Discretionary Ligatures.
To the best of my knowledge, neither Arial nor Trebuchet MS have that ligature in the glyph set, however, so there is something else going on. Perhaps a font substitution applied though a character style, maybe as a GREP style.
Peter's response is correct. There are some discretionary ligatures such as ‘ct’, ‘ch’, and ‘st’ that connect as you describe.
But none of these glyphs are available in any of the versions of Arial or Trebuchet that are normally bundled with Microsoft operating systems or Office products.
However, these discretionary ligatures are available in Calibri, one of the newer Microsoft fonts.
If you are seeing these discretionary glyphs appearing, somehow you are invoking, possibly by a character style or a default style Calibri (also a sans serif font like Trebuchet and Arial) with discretionary ligatures enabled.
In Adobe Indesign (select all the text/rectangle that using the same font) then go to Paragraph options and change "Adobe World-ready paragraph composer" change it to "Adobe Paragraph composer".