A few thoughts.
I find WILLIAM GRAYSON in convoy HX-283, not HX-293.
Look at the record for HX-283 in ConvoyWeb (go to http://convoyweb.org.uk/hx/index.html, then in the white box in the upper left scroll down to HX.283). The second column of the table listing the convoy participants is headed "Pdt." The two- and three-digit numbers in this column indicate each ship's station number, or physical position within the convoy. The first digit, or the first two digits in a three-digit number, indicates the column of the convoy in which the ship sailed, while the last digit is the row within that column. A careful examination of the numbers indicate the convoy had 13 columns and 6 rows, although not all columns had a full 6 rows.
Note that RIVERVIEW PARK was in position 32 (3rd column, 2nd row) while WILLIAM ASHLEY was in position 42 (4th column, 2nd row). That is, they were in adjacent columns and abreast of one another at some distance. (I forget the standard spacing in a convoy but it was several hundred yards between ships both laterally and fore and aft.) So these two ships were relatively close to one another, meaning there was a somewhat elevated risk of collision between the two.
By contrast, WILLIAM GRAYSON was in position 132 (13th column, 2nd row), therefore well away from the other two ships, probably several miles away, and unlikely to encounter either of the other two ships in a collision. My guess is that RIVERVIEW PARK and WILLIAM ASHLEY collided with one another but neither collided with WILLIAM GRAYSON, if all three ships maintained their stations. Possibly WILLIAM GRAYSON collided with yet another ship but that other ship was not so badly damaged as to require a return to port. Or possibly there was some unusual situation in which WILLIAM GRAYSON in fact ended up close enough to RIVERVIEW PARK or WILLIAM ASHLEY to result in a collision. But aside from the station numbers (and assuming they are correct), all of the preceding is pure speculation.
Also, for what it's worth, apparently your grandfather's service in JAMES TURNER preceded that of WILLIAM GRAYSON. I found your grandfather aboard JAMES TURNER at least in the period July-November 1943, while you know he was aboard WILLIAM GRAYSON at least as of March 1944 in convoy HX-283.
Finally, you may be interested to know that while WILLIAM GRAYSON was under construction at Bethlehem-Fairfield Shipyard in Baltimore in 1942, one of the other ships then under construction in the same yard was SS JOHN W. BROWN. The latter ship is still in existence and is operable, based in Baltimore, serving as a museum ship. I am, in fact, a member of its volunteer operating crew. If you are within some reasonable distance from Baltimore you may want to consider visiting the ship at dockside, or participating in one of our "Living History Cruises," which are six-hour excursions on Chesapeake Bay. You would be able to see first hand a virtual twin of WILLIAM GRAYSON and JAMES TURNER and, to a small extent, understand what your grandfather's experiences were like. If you ever visit, let me know in advance and I'll give you a personal tour. See www.ssjohnwbrown.org for information.